Arsenic Toxicity, Health Hazards and Removal Techniques

from water: an overview

Thomas S.Y. Choonga, T.G. Chuaha*, Y. Robiaha, F.L. Gregory Koaya, I. AznibaDepartment of Chemical and Environmental Engineering, bWater Technology Centre, Faculty of Engineering,

Universiti Putra Malaysia, Serdang 43400, Selangor, MalaysiaTel. +60 (3) 8946 6288; Fax: +60 (3) 8656 7120; email: [email protected]

Received 9 August 2005; Accepted 28 January 2007

Abstract

Arsenic contamination in water, especially groundwater, has been recognized as a major problem of catastrophicproportions. The toxicology and health hazard also has been reported for many years. Because of the recognition thatarsenic at low concentrations in drinking water causes severe health effects, the technologies of arsenic removal thathave become increasing important. The current regulation of drinking water standard is become more stringent andrequires arsenic content to be reduced to a few parts per billion. There are several treatment methods capable of thislevel of performance membranes, coagulation, anion exchange, disposable iron media, softening etc. Treatmentcost, operational complexity of the technology, skill required to operate the technology and disposal of arsenicbearing treatment residual are factors should be considered before treatment method selection. This paper aims toreview briefly arsenic toxicology and hazards and also the previous and current available technologies that have beenreported in arsenic removal. Residual generation and disposal after treatment will also be discussed.

Keywords: Arsenic; Toxicology; Membrane; Adsorption; Precipitation; GFH; Residual disposal

1. Introduction

Arsenic is a heavy metal with a name derivedfrom the Greek word arsenikon, meaning potent.The elements occur in environment in differentoxidation states and form various species, e.g., Asas As(V), As(III), As(0) and As (-III). In oxidized

*Corresponding author.

environment As appears mostly as oxyanions [1].Arsenic cannot be easily destroyed and can onlybe converted into different forms or transformedinto insoluble compounds in combination withother elements, such as iron. Many impuritiessuch as lead, iron and selenium may be mixed uptogether with arsenic wastes and make it uneco-nomical to remove.

Arsenic toxicity, health hazards and removal techniques

Desalination 217 (2007) 139166

0011-9164/07/$ See front matter 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2007.01.015

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Inorganic arsenic generally exists in two pre-dominant oxidation states, arsenite (NaAsO2) andarsenate (Na 2HAsO4), both of which are toxic toman and plants. Inorganic arsenic is alwaysconsidered a potent human carcinogen, associatedwith increased risk for cancer of the skin, lungs,urinary bladder, liver and kidney [2]. Arseniccommonly present in water are pH dependantspecies of the arsenic (H3AsO4) and arsenous(H3AsO3) acid systems respectively. These anionshave acidic characteristics, and the stability anddominance of a specific species depend on the pHof the solution, as shown in Fig. 1. Arsenates arestable under aerobic or oxidizing conditions,while arsenites are stable under anaerobic ormildly reducing conditions.

The presence of arsenic in natural water isrelated to the process of leaching from the arseniccontaining source rocks and sediments [4, 5]. It isgenerally associated with the geochemical envi-ronments such as basin-fill deposits of alluvial-lacustrine origin, volcanic deposits, inputs fromgeothermal sources, mining wastes and landfills[6,7]. Arsenic is a constituent of over 300 mine-rals and is commonly found in non-ferrous oressuch as copper, lead, zinc, gold and uranium.Arsenic is a primary constituent of certain ores(for example the copper mineral enargite) andoccurs as a trace impurity in others [8]. Uncon-trolled anthropogenic activities such as smeltingof metal ores, use of arsenical pesticides andwood preservatives agents may also releasearsenic directly to the environment [9].

Presently, arsenic has been used for a varietyof purposes such as treatment of ulcers, tuber-culosis, syphilis, and many other ailments. Morerecently, arsenic has been used as an insecticide,fungicide, rodenticide, and wood preservative.The common application of arsenic are in themanufacture of pesticides (including wood pre-servatives), dessicants, glass, alloys, electroniccomponents (semiconductors), pigments, andpharmaceuticals [10].

Fig. 1. Potential-pH diagram for the arsenic-water systemat unit activity of all species [3].

In this work, the aim of this article is to pro-vide general description of the toxicity of arsenic,health hazards, previous and current literatures onthe techniques in treating or removing arsenic.Different techniques in removing arsenic viz.adsorption, ion exchangers, membrane, coagu-lation and precipitation will be discussed. The USEnvironmental Protection Agencys (EPA)Incineration Research Facility has carried outsome tests in evaluating the potential of incinera-tion as a treatment option for contaminated soilsby arsenic and lead [11]. Due to the volatilizationof arsenic containing compounds emission, theincineration is not practical for this purpose.Methods of arsenic wastes residual disposal aftertreatment will also be discussed.

2. Toxicity and reported health hazards

Arsenic contamination in natural water is aworldwide problem and has become an importantissue and challenge for the world engineers,scientists and even the policy makers. For exam-ple, chronic arsenic toxicity due to drinking

T.S.Y. Choong et al. / Desalination 217 (2007) 139166 141

arsenic-contaminated water has been one of theworst environmental health hazards affectingeight districts of West Bengal since the early1980s. Detailed clinical examination and inves-tigation of 248 such patients revealed proteanclinical manifestations of such toxicity. Over andabove hyperpigmentation and keratosis, weak-ness, anaemia, burning sensation of eyes, solidswelling of legs, liver fibrosis, chronic lungdisease, gangrene of toes, neuropathy, and skincancer are some of the other manifestations [13].It has also been reported in recent years fromseveral parts of the world, like USA, China,Chile, Bangladesh, Taiwan, Mexico, Argentina,Poland, Canada, Hungary, Japan, and India [4,1420]

The World Health Organization (WHO)revised the guideline for arsenic from 0.05 to0.01 mg/L in 1993 [21]. As the results, Germanyhas lowered its permissible limit of arsenic to0.01 mg L!1 in 1996 [22], while the Australiandrinking water limits were also lowered from0.050 to 0.007 mg/l. The French current standardis 0.015 mg/L, Vietnam and Mexican standard is0.05 mg/L [23]. In the European Union, thearsenic standard level is now set to 10 g L!1.The EPA has also eventually implemented thereduction of permissible values of arsenic indrinking water from 50 to 10 g/L in light of theepidemiological evidence to support the carcino-genic nature of the ingested arsenic and itsconnection with liver, lung and kidney diseasesand other dermal effects [http://www.epa.gov/safewater/arsenic final rule.html, Arsenic indrinking water rules (66FR 6976, 22 January2001)]. Many US water utilities protested theEPA adoption of the WHO recommendation torevise the arsenic standard of 10 ppb. Naturallyoccurring arsenic, adsorbed from rocks throughwhich water passes, is present in some 4,000 sitesin the US, mainly in the southwest and northeaststates. Utilities supplying water complied withearlier EPA standards of a 50-ppb maximum

contaminant level (MCL), but the revised com-pliance levels that reduced this to 10 ppb MCLrepresented a big change.

The Malaysia Environmental Quality Act(1974) has stated that limit of sewage and indus-trial effluents for arsenic ranged between 0.050.1 mg/L. However, an overview of groundwatercontamination in Malaysia has been reported bySuratman [24]. The Department of Environment,Malaysia, monitors a programme at areas that arepotentially exposed to contamination that includeindustrial, animal burial, rural, urban/suburban,and agricultural area, golf courses and landfills,and has found that in general mercury, arsenic,phenolic compounds and nitrate exceeded bench-mark values for agricultural, landfill and indus-trial sites.

Jain and Ali [25] reported comprehensively onthe occurrence and toxicity of arsenic. The toxi-cology of arsenic is a complex phenomenon andgenerally classified into acute and sub-acutetypes. The acute arsenic poisoning requiringprompt medical attention usually occurs throughingestion of contaminated food or drink. Themajor early manifestation due to acute arsenicpoisoning includes burning and dryness of themouth and throat, dysphasia, colicky abnormalpain, projectile vomiting, profuse diarrhea, andhematuria. The muscular cramps, facial edemaand cardiac abnormalities, shock can developrapidly as a result of dehydration [26].

In general, there are four recognized stages ofarsenicosis, or chronic arsenic poisoning [23,27]:C Preclinical: the patient shows no symptoms,

but arsenic can be detected in urine or bodytissue samples.

C Clinical: various effects can be seen on theskin at this stage. Darkening of the skin (mela-nosis) is the most common symptom, oftenobserved on the palms. Dark spots on thechest, back, limbs or gums have also beenreported. Oedema (swelling of hands and feet)is often seen. A more serious symptom is


keratosis, or hardening of skin into nodules,often on palms and soles. WHO estimates thatthis stage requires 510 years of exposure toarsenic.

C Complications: clinical symptoms becomemore pronounced and internal organ areaffected. Enlargement of liver, kidneys andspleen have been reported. Some researchindicates that conjunctivitis (pinkeye), bron-chitis and diabetes may be linked to arsenicexposure at this stage.

C Malignancy: tumors or cancers (carcinoma)affect skin or other organs. The affected per-son may develop gangrene or skin, lung orbladder cancer.

The results of clinical findings for arsenicpoisoning from drinking arsenic contaminatedwater show the presence of almost all the stagesof arsenic clinical manifestation [28]. Diseasescaused by arsenic poisoning are no longer newsbut reported worldwide. In Antofagasta, Chile,over 12% of the population exhibiting dermato-logical manifestations related to arsenic due toconsumption of high arsenic containing drinkingwater [29].Exposure to arsenic via drinking water(groundwater) has been reported to cause a severedisease of blood vessels leading to gangrene,known as blackfoot disease:, in Taiwan [30].According to some estimates, arsenic in drinkingwater will cause 200,000270,000 deaths fromcancer in Bangladesh alone [31]. West Bengaland India have long known suffered from theproblem of arsenic contaminated groundwaterand claims as the biggest calamity in the world[1820].

All these cases have provided hints on theclose relationship between the prevalence ofcutaneous lesions and the exposure to drinkingwater containing high levels of arsenic. Karim[32] reported the data of concentration of arsenicin urine, hair and nails of the affected people indifferent arsenic contaminated water ingestion,including Bangladesh (Table 1). Other minor

Table 1Concentration of arsenic in urine, hair and nails of theaffected people in different arsenic contaminated wateringestion episodes [32]

Location Conc. inurine(mg/l)

Conc. inhair(mg/kg)

Conc.in nails(mg/kg)

Fairbanks, AL 0.1783 1.0 4.0

Millard County,UT

0.025-0.66 0.104.7

Antofagasta,Chile

0.025-0.77 4.083.4

Lassen County,CA

0.012.0

Taiwan 0.03660.259 West Bengal,India

0.032.0 1.8131.05

1.4752.03

Bangladesha 0.059.42 1.119.84

1.333.98

aAverage of the few data available in affected areas.

incidents of arsenic poisoning from groundwater,also have been reported from Minnesota, USA[33], Millard County, Utah [34,35], Ontario,Canada [36], Nova Scotia, Canada [37], NewZealand [38], Nakajo, Japan [39]. Table 2 showsthe distribution of arsenic drinking water concen-trations from at Millard County, Utah.

3. Treatment and removal of arsenic

3.1. Treatment of arsenic

The relationship between surface-source andfinished water quality, in its simplest form, is thatcleaner source water requires less intense watertreatment and has lower associated acute andchronic health risks. Common health risks ofdrinking water include enteric pathogens, disin-fection by-products, chemical contamination, andother toxic compounds. There is little commercial


Table 2Distribution of arsenic drinking water concentrations from historical and recent arsenic measurement data for Utahcommunities in the study area [35]

Town Number Median Mean Minimum arsenicconc., ppb

Maximum arsenicconc., ppb

Standarddeviation

Hinckley 21 166 164.4 80 285 48.1Deseret 37 160 190.7 30 620 106.6Abraham 15 116 134.2 5.5 310 67.2Sugarville 6 92 94.5 79 120 15.3Oasis 7 71 91.3 34 205 57.8Sutherland 19 21 33.9 8.2 135 31.8Delta 46 14 18.1 3.5 125 17.7

interest in investing in plants and technology torecover arsenic and its compounds when there isa very limited market for the recovered material(except in a relative high purity). Safety in hand-ling and storage has made it even less practical torecover the arsenic. Dilution and dispersionmethods, however, may attract the interest ofmining and waste disposal operators. The methodprovides the possibility for combining numerouswaste streams together and in a way which dilutesthe hazardous contaminants, thus passing anyregulatory limits. It helps in reducing humanexposure to arsenic. This is, in technical practice,not solving the problem by all mean, but mainlyserved as a legislative solution [12].

Conventionally, there are several methods forarsenic removal. These methods include coagu-lation and flocculation, precipitation, adsorptionand ion exchange, membrane filtration Alter-native methods like ozone oxidation, bioremedi-ation and electrochemical treatments also used inthe removal of arsenic. Each method will bedescribed briefly together with the related worksin the following sections.

3.2. Coagulation and flocculation

In arsenic removal processes, coagulation andflocculation are among the most common methodemployed. Although the terms coagulation and

flocculation are often used interchangeably orthe single term flocculation is used to describeboth they are, in fact, two distinct processes.Coagulation is the destabilization of colloids byneutralizing the forces that keep them apart.Cationic coagulants provide positive electriccharges to reduce the negative charge (zeta poten-tial) of the colloids. As a result, the particlescollide to form larger particles. Rapid mixing isrequired to disperse the coagulant throughout theliquid. Flocculation is the action of polymers toform bridges between the larger mass particles orflocs and bind the particles into large agglo-merates or clumps. Bridging occurs when seg-ments of the polymer chain adsorb on differentparticles and help particles aggregate. An anionicflocculant will react against a positively chargedsuspension, adsorbing on the particles and caus-ing destabilization either by bridging or chargeneutralization.

Aluminium-based coagulation with disinfec-tion by chlorination is one of the commonly usedtreatment methods. McNeill and Edwards [40]reported a wide range in decreases in solubleAs(V) concentrations for five alum coagulationtreatment plants (674%). Gregor [41] studied thechanging forms and concentrations of arsenicthrough aluminium-based coagulation treatmentprocesses drinking-water treatment plants thatabstract water from the river. His findings


Fig. 2. (a) SEM micrographs of akaganite particles and (b) SEM micrographs of akaganite particles after As(V) sorption[89].

provide some insights into the critical steps ofcoagulation. For aluminium-based coagulationwith disinfection by chlorination, the form ofarsenic most likely to be present in the treatedwater is soluble As(V) because final chlorinationshould have converted any remaining As(III) toAs(V). His finding also showed that pre-chlorination can have an adverse effect on otherwater quality parameters such as the formation ofdisinfection by-products and the release of tasteand odour compounds from algal cells.

Several feasibility studies have been carriedout using a waste material in the treatment ofarsenical wastewater. Soner Altundoan andTmen [42] studied the As(V) removal usingneutralization of liquid phase red muds (LPRM)-arsenical solution mixtures with acid solutionaccompanied with air-agitation and neutralizationof those mixtures with CO2 gas. They studied theeffect of LPRM/(As(V) solution) volumetric ratioon the removal of As(V) by co-precipitationarsenic together with aluminium present as alu-minate in the LPRM. It is found that As(V) wasremoved effectively by LPRM with a volumetricLPRM/(As(V) solution) ratio of 0.1 from anarsenical solution in the As(V) concentration of20 mg dm!3. For an efficient removal, it was

found that a Al/As(V) molar ratio of 68 wasrequired.

Ferric salts are common in the uses of as acoagulant. Yuan et al. [43] studied a combinationsystem of ferric sulphate coagulation/sand filtra-tion in arsenic removal. The method is economicand effective. Zouboulis and Katsoyiannis [44]studied arsenic removal by applying a modifi-cation of a conventional coagulation/flocculationprocess. The modifications refer to the intro-duction of pipe flocculation process in the firststage of the technique, whereas the second stephas been performed by direct filtration with sandfilters, instead of separation by sedimentation. Intheir system, alum or ferric chloride was used asthe coagulant agent enhanced by some organicpolymers. The coagulants were found to beefficient regarding arsenic removal and hadachieved up to 99% of arsenic removal. Karcheret al. [45] and Guo et al. [46] also reported the theuses of ferric chloride and lime-polyferric sulfateas the coagulants. Han et al. [47] used ferricchloride and ferric sulphate as flocculants inarsenic removal. The results have shown a signi-ficant arsenic removal through adsorptionmechanism onto the ferric complexes present.Wickramasinghe et al. [48] also studied the


application of ferric based coagulants in treatingthe city groundwater that has been contaminatedby arsenic. The results of the bench-scale experi-ments conducted indicate that coagulation withferric ions followed by filtration is effective inreducing arsenic concentration in the water tested.However, the actual efficiency of removal ishighly dependent on the raw water quality.

3.3. Adsorption and ion exchangers

Adsorption is a process that uses solids forremoving substances from either gaseous orliquid solutions. Adsorption phenomena are ope-rative in most natural physical, biological, andchemical systems. Adsorption operations employ-ing solids such as activated carbon, metalhydrides and synthetic resins are used widely inindustrial applications for purification of watersand wastewaters. The process of adsorptioninvolves separation of a substance from one phaseaccompanied by its accumulation or con-centration at the surface of another. Physicaladsorption is caused mainly by van der Waalsforces and electrostatic forces between adsorbatemolecules and the atoms which compose theadsorbent surface. Thus adsorbents are charac-terized first by surface properties such as surfacearea and polarity.

For arsenic removal, an ion exchange resin,usually loaded with chloride ions at theexchange sites, is placed in vessels. The arseniccontaining water is passed through the vesselsand the arsenic exchanges for the chloride ions.The water exiting the vessel is lower in arsenicbut higher in chloride than the water entering thevessel. Eventually, the resin becomes exhaus-ted; that is, all or most of the exchange sitesthat were loaded with chloride ions becomeloaded with arsenic or other anions. The chlorideions that used to be on the resin were exchangedfor the arsenic and other anions that were in thewater being treated.

The effect of the presence of sulfate, com-petition with other anions, is an important factorto ion exchanger treatment of arsenic. Sulfatelevers can limit the applicability of ion-exchangeras arsenic treatment. Jackson and Miller [49]reported that sulfate was reported not to influenceAs(V) sorption by ferrihydrite but resulted in aconsiderable decrease in As(III) sorption belowpH 7, with the largest decrease at the lowest pH.Sorbed As(V) by ettringite [Ca6Al2(SO4)3(OH)1226H2O] was also not desorbable in the presenceof concentrated sulfate and high ionic strengthsolutions [50]. On the contrary, sulfate was foundto decrease both As(V) and As(III) sorption onhydrous ferric oxide in the pH range of 47 [51].Disagreement in the literatures on the effects ofsulfate on As(V) and As(III) sorption may havederived from different experimental conditions.

Scattered research has already been conductedon a wide variety of sorbents. Some of thereported sorbents include zeolites, goethite, clay,kaolinites, activated carbon, chitosan beads, coco-nut husk, coal, fly ash, ferrous iron, alumina,zirconium oxide, red mud, petroleum residues,rice husk, human hair, sawdust, manganesegreensand, orange juice residues, akaganite-nanocrystal and chome waste.

Activated carbon is also commonly used as thematerial in arsenic treatment [5254]. Eguez andCho [55] measured the adsorption of As (III) andAs (V) using activated carbon at various pHvalues. From the effect of temperature on adsorp-tion, they could determine the isoteric heat ofadsorption. Other researchers [53,56] impreg-nated carbon with various metal ions such as ironoxide in order to improve arsenic adsorption. Theiron oxide impregnated activated carbon hasshown higher As(III) and As(V) removals com-pared with the non-impregnated carbon.

Rajakovic [57] found that carbon pretreatedwith Ag+ or Cu2+ ions improved As(III) adsorp-tion but reduced As(V) adsorption. Evdokimov etal. [58] reported that arsenic adsorption can be


improved by impregnating carbon with ferrichydroxide or tartaric acid. Rajakovic andMitrovic [59] showed that chemically treatedactivated carbon exhibits high adsorption capa-city for arsenic.

Lorenzen et al. [8] studied the factors (such assolution pH, carbon type and carbon pretreatmentand elution of the arsenic from loaded carbon)that affect the mechanism of the adsorption ofarsenic species on activated carbons. They foundthat As(V) is more effectively removed fromsolution by using activated carbon with high ashcontent and pre-treatment of the carbon withCu(II) solutions improves its arsenic removalcapacity.

In these studies, commercially available acti-vated carbons were used. The use of commercialactivated carbon is not suitable for developingcountries because of its high cost. The prepa-ration of low cost adsorbent for water purificationand wastewater treatment has been reviewed byPollard et al. [60] and Bailey et al. [61]: agricul-tural wastes like rice husk [62,63], coconut husk[64], amine modified coconut coir [65], car-bonised wood powder [66], sawdust [67], orangejuice residues [68] and waste tea fungal biomass[69].

Iron oxides also have been widely used assorbents to remove contaminants from waste-water and liquid hazardous wastes compared toactivated carbon. Removal has been attributed toion exchange, specific adsorption to surfacehydroxyl groups or coprecipitation. Hydrousferric oxide (HFO) is an important sorbent inwastewater treatment especially for hazardouschemical. Olivier et al. [70] removed arsenicgroundwater by filtering the water through sandand zero-valent iron. As(V) sorbed on the form-ing hydrous ferric oxides (HFO) resulted from theoxidation of iron.

Different similar sorbent materials have beenalso used, including amorphous iron hydroxide[71] and ferric hydroxide [7274]. Other types of

ferric products, such as ferrihydrite [75], silicathat containing iron (III) oxide [76], iron-oxideimpregnated activated carbon [77], Ce(IV)-dopediron oxide [78], iron oxide-coated sand [79],iron(III)-Poly(hydroxamic acid) complex [80],ferric chloride [[81], Fe(III)-doped alginate gels[82], nanocomposite adsorbent based on silicaand iron(III) oxide [83], and iron oxide-coatedpolymeric materials [84] are also used in arsenictreatment.

Arsenic removal technology by adsorptionwith a commercial granular ferric hydroxide(GFH) has been developed in the early 1990s[73,85]. It can be applied in simple fixed bedreactors, similar to those for activated alumina oractivated carbon. Simplified operation is a keybenefit of the system, which will operate withoutthe need for chemical pre-feed or pH correction.GFH has a high adsorption capacity in naturalwaters. The work, carried out by Driehaus et al.[73], shows that GFH possesses high treatmentcapacity of 30,00040,000 bed volumes. Jekeland Seith [86] compared the methods for theprecipitation/flocculation by iron (III)-chlorideand iron (II)-sulphate as well as adsorption onGFH in a full scale water treatment plant. Theirfindings also show that adsorption on granulatediron hydroxide has proven to be the methodwhich will provide greater operational reliabilitywith least maintenance and monitoring efforts.Ruhland and Jekel [87] had evaluated threearsenic treatment techniques: direct filtration withFeCl3, adsorptive filtration with FeSO4 andadsorption on granulated ferric hydroxide. Theadsorption on granulated ferric hydroxide isfound to be most preferential process for arsenicremoval on the tested conditions.

Earlier research achieved promising results ona small scale tests in Germany using a granularform of ferric hydroxide as an adsorption med-ium. A follow-up resulted in a cooperationagreement with German chemical company,Bayer AG, which developed a totally new granu-


Fig. 3. Schematic diagram of SORB 33 standard process[88].

lar ferric oxy hydroxide in partnership withSevern Trent Water of Fort Washington, Penn-sylvania, USA. Under this collaboration, SORB33 and Bayoxide E33, a combination of adsorp-tion system and ferric oxy hydroxide mediumspecifically designed for arsenic removal aredeveloped [88]. Fig. 3 shows a schematic diagramof the SORB 33 standard process. It is claimed bythe developer that arsenic removal can beadsorbed and removed to a level below the drink-ing water standard of 10 g/L. With SORB 33,the only factor which needs monitoring is thepressure drop of water through the adsorbent bed,which can be done remotely. However, underhigh pH conditions high levels of vanadium,phosphate and silica can reduce the adsorption ofarsenic, requiring more frequent changing.

Deliyanni et al. [8991] synthesized a noveladsorbent, akaganite-type -FeO(OH) in thelaboratory by precipitation from aqueous solutionof Fe(III) chloride and ammonium carbonate for

arsenic removal. Advantage of this sorbent,which found to be nanostructured, was its highsurface area and narrow pore size distribution[89]. On the other hand, the sorbent retained itshigh surface area and crystalline for long andeven after its regeneration. The maximum loadcapacity was found to be about 100120 mgAs(V) per g of akaganite, when 0.5 g l!1 akaga-nite was used at 298 K, which is higher in com-parison with other sorbents, like hydrous ironoxides, ferrihydrite and goethite.

Lenoble et al. [92] used the synthesised iron(III) phosphate to remove arsenic from water.Results showed that adsorption capacities werehigher towards As (III), leading to Fe2+ andHAsO42! leaching. The high release of phosphateand iron will exclude its application in drinkingwater plants is the main drawback for this sorbentto be used in waste water treatment.

Other reported works on the metal oxide basedadsorbents include manganese oxide [9294],zirconium oxide [9599] and alumina [100105].Most of these studies are carried out in low con-centration of arsenic solution or batch experi-ments. Surtherland and Woolgar [106] comparedthe treatment methods on arsenic removal usingadsorption (Alcan enhanced activated alumina),oxidation and co-precipitation, chemical oxida-tion and adsorption. They found that all tech-nologies that remove arsenic from groundwaterwill at some time produce arsenic waste either asa solid or a liquid waste sludge. Tatineni Balaji etal. [107] evaluated the uses of zirconium (IV)loaded chelating resin (Zr-LDA) with lysine-Na,Na diacetic acid functional groups for theremoval of As(V) and As(III). From their findingin column adsorption, the adsorption of As(V) ismore favorable compared to As(III), due to thefaster kinetics of As(V) compared to As(III).Similar studies were also carried out by Suzuki etal. [97].

Zeolites have also attracted ever increasinginterest from academic and industrial laboratories.


They represent an important group of materialsdue to their catalytic, sieve and exchange proper-ties. Works reported on zeolites used as adsorbentare aluminium-loaded Shirasu-zeolite [108,109],clinoptilolite and chabazite zeolites [22,110].Other reported low cost adsorbents include clay[111], kaolin [112], goethite [113115], fly ash[116], red mud [117], humic acid [118], humanhair [119], hematite/feldspar [120] and fungalbiomass [121].

3.4. Membrane filtration

Membrane separation is addressed as a pres-sure driven process. Pressure driven processes arecommonly divided into four overlapping cate-gories of increasing selectivity: microfiltration(MF), ultrafiltration (UF), nanofiltration (NF) andhyperfiltration or reverse osmosis (RO).MF canbe used to remove bacteria and suspended solidswith pore sizes of 0.1 to micron. UF will removecolloids, viruses and certain proteins with poresize of 0.0003 to 0.1 microns. NF relies onphysical rejection based on molecular size andcharge. Pore sizes are in the range of 0.001 to0.003 microns. RO has a pore size of about0.0005 microns and can be used for desalination.High pressures are required to cause water to passacross the membrane from a concentrated todilute solution. In general, driving pressureincreases as selectivity increases. Clearly it isdesirable to achieve the required degree of sepa-ration (rejection) at the maximum specific flux(membrane flux/driving pressure). Separation isaccomplished by MF membranes and UF mem-branes via mechanical sieving, while capillaryflow or solution diffusion is responsible forseparation in NF membranes and RO membranes[122].

Nanofiltration is considered as one of themethods that can be used to meet regulations forlowered arsenic concentrations in drinking water[123]. Waypa et al. [124] studied the arsenic

removal from synthetic freshwater and fromsurface water sources by NF and RO. The resultsshow that both As(V) and As(III) were effectivelyremoved from the water by RO and NF mem-branes (NF70, Dow/Filmtec) over a range ofoperational conditions. Both membranes canachieve rejections of 99%. Removal of As(V) andAs(III) was comparable, with no preferentialrejection of As(V) over As(III). This suggests thatsize exclusion governed their separation behav-iour and not the charge interaction. Urase et al.[125] studied the pH effect on the rejectionchange and explained with the extended Nernst-Planck equation which showed that electricallycharged membranes generally have a higherrejection for charged solutes than for non-chargedsolutes. Vrijenhoek and Waypa [126] also inves-tigated the behaviour of the membrane. They alsofound that it is consistent with the extendedNemst-Planck equation model predictions for anuncharged membrane where size exclusioncontrols ion retention. However, separation ofarsenic species was a due to a combination of sizeexclusion, preferential passage of more mobileions, and charge exclusion. Kouti et al. [127]showed that the membrane material and the mem-brane pore size distribution influence the un-charged organic molecules rejections.

Saita et al. [128] studied the effects ofoperating conditions in removal of arsenic fromwater by nanofiltration. Their findings show thatarsenic rejection was independent of trans-membrane pressure, crossflow velocity andtemperature. The co-occurrence of dissolved inor-ganics does not significantly influence arsenicrejection.

Seidel et al. [129] used loose (porous) NFmembranes to study the difference in rejectionbetween As(V) and As(III). The rejection ofAs(III) was below 30% and was much lower thanthe rejection of As(V). The removal of As(V) wasvaried between 60% and 90%. Oh et al. [130]studied the feasibility of removing the arsenic by


a low pressure NF that was applied in rural areaswith an electricity supply shortage. The NF isoperated by using just a manually operatedbicycle pump. Sato et al. [131] also investigatedthe performance of nanofiltration for arsenicremoval. In their studies, both As(V) and As(III)removal by NF membranes was not affected bysource water chemical compositions. NF mem-branes could remove over 95% of pentavalentarsenic. Furthermore, more than 75% of trivalentarsenic, which is toxic form of arsenic, could beremoved without any chemical additives. Anoverview on the nanofiltration is also discussedby Bruggen and Vandecasteele [132].

Reverse osmosis (RO) membranes have beenidentified as another alternative to remove arsenicin water. Kang et al. [133] had studied the effectof pH on removing of arsenic using reverseosmosis. They found that the removal of arseniccompound is almost proportional to the removalefficiency of NaCl. The removal of As(V) ismuch higher than As(III) over the pH range 310.The effect of solution pH on the removal ofarsenic using RO membranes was stronglyaffected by the solution pH, especially As(IlI).Ning [134] had reviewed the removal mechanismof RO and concluded that arsenic in the com-monly high oxidation states of (V) is very effec-tively removed by RO. With further attention tothe removal of the weakly acidic arsenic (III)species in waters by the operation of RO atsufficiently high pHs made possible by the newerantiscalants, practical processes can be developedwith RO to remove all major species of arsenicfrom water.

Han et al. [47] had studied the feasibility ofcombination of flocculation and microfiltrationfor arsenic removal from drinking water. Micro-filtration of the flocculated water had resulted inrejection of the floes formed by the membranethus leading to low turbidity and arsenic removalin the filtrate. However, with addition of smallamounts of cationic polymeric flocculant can

greatly increase the permeate flux duringmicrofiltration.

Shih [135] has illustrated an overview ofarsenic removal on pressure driven membraneprocesses. In his work, he explored the para-meters that may influence the arsenic removalefficiency by membrane technologies such assource water parameters, membrane material,membrane types and membrane processes.Brandhuber and Amy [136] have also carried outan intensive study on arsenic removal fromdrinking water using several membrane filtrationmethods via bench and pilot testing. Theysummarized the guidelines of selection for arsenicremoval via membrane treatment as tabulated inTable 3. A few important findings from theirworks are RO membranes or tight NF membranesappear to be able to sustain high rates of arsenic.Coagulation, as a pretreatment, can be coupledwith membranes of relatively large pore size toobtained substantial arsenic removal. However,greater coagulant doses will be required thencompared to As (V).

Preoxidation of As(III) to As(V) followed byNF may achieve high rates of arsenic removal. Ifarsenic is present in the particulate form, mem-branes of relatively large pore size may beeffective for arsenic removal.

The drawbacks of using of membrane inarsenic removal are:C the systems are more costly than other

treatment methodsC the discharge of the concentrate can be a

problemC water loss associated with concentrate stream

membrane fouling and flux decline

The membrane technology is very little usedwhen the objective is to remove only the arsenic,and when this element is the only one contami-nant in the raw water. The membranes arejustified when the total dissolved solids due to thepresence of sulphates, nitrates, carbonates etc., is


Table 3Treatment option for various arsenic bearing sources water characteristics [136]

Source water Treatment option Possible treatment

Filtration alone

Characteristic RO NF UF MFa Preoxidationb

As speciationAs(III)As (V)

RR

PER

NRPE

NRNR

RNR

As size distributionDissolvedParticulate

RNR

PENR

NRPE

NRPE

NRNR

Co-occurrenceNOMInorganic

PER

PEPE

NRNR

NRNR

NRNR

R, recommended; NR, not recommended; PE, possibly effective.aRemoval of other arsenic forms possible with ferric coagulants.bPreoxidation is considered as a pretreatment.

important and require a treatment. In practice, thein-line coagulation used before a membrane treat-ment (MF or UF) provides very good perfor-mances. The coagulant plays the role of adsorbentand the membrane plays the role of physicseparator [137,138]. Several issues, however, stillremain to be resolved before chemical pre-treatment like coagulation can be applied opti-mally in the water treatment membrane field.These issues include the impact of chemical pre-treatment on the performance of membrane sys-tems (i.e., membrane reversible fouling, chemicalcleaning frequency), the compatibility of thesechemicals with membrane materials, the optimumconditions for chemical pre-treatment, and overallcost and benefits of chemical pre-treatment to MFand UF membrane systems [139].

Recent advanced in membrane technology inarsenic removal including electro-ultrafiltration(EUF) [140]. EUF is found to possess goodpotential in treating arsenic from water. Thetraditional 100 kDa UF membrane is unable toremove As(III) or As(V) from water. After apply-ing electricity to UF, As(V) rejection increaseddramatically. The removal mechanisms adopted

for As(V) were relied on electrophoretic forceand electrochemical reduction.

3.5. Precipitation processes

Four precipitation processes are useful; alumcoagulation, iron coagulation, lime softening, anda combination of iron (and manganese) removalwith arsenic.

3.5.1. Alum precipitationAlum precipitation is able to remove solids

and dissolved metals. For the removal of arsenic,alum is most effective if an oxidizing agent, suchas chlorine, is added ahead of the flocculator andclarifier and the pH is reduced to 7 or less. Itwould probably be necessary to use a number ofchemicals in order to treat the arsenic in thedrinking water. The arsenic removed from thewater would be contained in the alum sludgefrom tile clarifier [123].

3.5.2. Iron precipitationThe most outstanding attributes of this

technology are its simplicity, versatility, selec-


tivity, and low cost. The operator merely adjustspH and iron dose to remove the trace elements ofchoice to the desired extent. In this process, aniron compound, such as a ferric salt (for example,ferric chloride or ferric sulphate), is added to theuntreated water. The arsenic combines with theiron to form a precipitate (iron oxyhydroxide inthe form of sludge) that settles out in the clarifier.Following the clarifier, a filter is employed whichremoves iron/arsenic particles not taken out in theclarifier. The best arsenic removal rates areobtained at pH of less than 8.5 with or withoutchlorine [123]. In practice ferric chloride is morefrequently used rather than ferric sulfate Jones etal. [141] had studied the removal of arsenic (V)from sulphuric solution. The found that ferrousiron provides an effective treatment giving resi-dual dissolved arsenic concentrations of below1.0 ppm over a range of compositions along with99.9% removal. They also noticed that ferric irontreatment is more effective in conjunction withmixed lime and magnesium hydroxide.

3.5.3. Lime softeningIt is well known that lime softening will

remove substances from water other than hard-ness (calcium and magnesium ions). Arsenic, too,can be removed by lime softening. However, thelime softening technology is justified when asoftened water is required. The produced sludgedoes not present any added value, and can limitthe use of this technology. For this case, thesuppliers prefer a treatment in two stages: remo-val of arsenic following with a lime softening[138]. The arsenic removal efficiencies of thelime softening process are significantly affectedby the pH and the presence (or absence) ofchlorine. Chlorine is required to oxidize thearsenic and acid would probably be necessary tolower the pH of the treated water to acceptabledrinking water levels. The arsenic removed fromthe water will be removed together with the limesludge produced by the process.

Several works have been reported for thismethod. Field et al. [142] had reported in detailon arsenic treatment for drinking water by limesoftening. Eberhard et al. [143] had patented themethod using lime participation to remove thearsenic from a sulphur dioxide-containing solu-tion resulting from scrubbing the flue gas in thesmelting facility. Huang and Rong [144] alsoreported the uses of calcium breach and lime inthe treatment of sewage with high arseniccontent.

3.5.4. Combined with iron (and manganese)removal

There are a number of processes that are usedto remove iron and/or manganese from water byoxidizing the iron and/or manganese from theirsoluble state (valence of 2+) to a higher valenceto form iron and/or manganese precipitates thatcan be filtered from the water. One of the pro-cesses involves a proprietary media. In one varia-tion of this process, chlorine is injected into theraw water containing iron and/or manganese andallowed to react with the iron and/or manganesein a reaction vessel for a short time a minute ortwo. Following the chlorine reaction vessel, sul-fur dioxide may also be injected into the waterand allowed to react for a short period of time.The water is then discharged into one or morefilter vessels which contain the proprietary media.

Sorg [145] has reported the important of themedia selection in arsenic removal in this process.Differences exist between media that result indifferent capabilities. Other factors that impactarsenic removal capacity using this process arepH in source water, competitive ligands such asSi, PO4, etc and concentration of As and otherligands. Sorg and Lytle [146] proposed arsenicselection guide based on Fe/As in source water(Fig. 4). The arsenic removal is increased wheniron oxide was added the treatment system(Fig. 5). Kunzru and Chaudhuri [147] reportedthat their findings from batch adsorption/


Fig. 4. Arsenic selection guide based on iron/arsenic in source water [146].

Fig. 5. Arsenic demonstration program, EPA at Climax, MN [146].


Table 4Summary of arsenic precipitation processes [123]

Parameters Alum Iron Lime softening Iron and Manganese

Chemicals Cl2AcidAllumNaOH

Cl2Fe2(SO4)3

Cl2LimeAcid

Cl2FeCl3 aSO2 aKMnO4 aPolymeric aluminium silicate sulfate (PASS)aOrganic polymer

pH 10.5 7+Removal, %

with Cl2w/o Cl2

9020

9060

9080

40-90

Initial arsenic conc., g/L 300 300 400


Table 5Performances and limitations of arsenic oxidants.

Oxidants Performances and Limitations of Arsenic Oxidants

Ozone (O3) Ozone may be the most satisfactory for pre-oxidation to convert As (III) to As(V) in waterwith the requirement to reduce disinfectant byproducts. Limitations are probably fewer on theuse of ozone as a pre-oxidant for arsenic than when O3 is used after filtration as a primarydisinfectant. Assimilable Organic Carbon (AOC) formation is ameliorated bycoagulation/filtration treatment downstream, and oxidation of bromide, though still possible,is much less likely during pre-oxidation because the development of a significant ozoneresidual is not necessarily required. Ozone pre-oxidation before nanofiltration could present aproblem if the AOC that is formed has a low molecular weight and passes through themembrane.

Hydrogen peroxide(H2O2)

Hydrogen peroxide oxidation was effective but limited by reactions with calcium hydroxide.After oxidation, the resulting arsenate waste was effectively stabilized using ferric sulfate.

Chlorine (Cl) Chlorine is a good oxidant for As(III), but application must come early in the treatment trainwhen disinfectant byproduct precursor concentration is high and there is a danger ofproducing large concentrations of disinfectant byproducts.

Permanganate Permanganate may work better than chlorine, however, no sufficient information on thepermanganate demand for arsenic oxidation relative to the demand exerted by othersubstances.

disinfecting harmful bacteria and/or pollutants, itgenerally leaves behind no by-products. Ozonewhen added to water which contains arsenic andsoluble iron, will oxidize both arsenic and iron,forming sites on the ferric hydroxide for arsenicto adsorb to. The arsenic bearing iron hydroxidecan then be removed by solid liquid separationprocesses.

Frank and Clifford [158] showed that underambient conditions, all the As(III) was com-pletely oxidized with oxygen and chlorine within61 days. Other researchers had investigated theoxidation of arsenic in the presence of oxygen[158160] and applying ozone [160]. However,information on the rates of arsenic oxidation ingroundwater and the specific rate constants ofthese studies are often inconsistent.

Kim and Nriagu [161] studied the rates ofoxidation of naturally occurring arsenic ingroundwater samples in the presence of ozone, airand pure oxygen gas. Air was used to assess theeffect of reduced partial pressure of oxygen on

the oxidation rate. The half-life for As(III) oxida-tion by ozone was very short, only 4 min. Thehalf-life for pure oxygen ranged from 2 days to 5days, and for air a half-life of 9 days. Theirfindings showed that ozone can be used toremove arsenic from groundwater through oxida-tion, coprecipitation and adsorption reactionseffectively. A comparison of the relative perfor-mance between ozone and other oxidants e.g.hydrogen peroxide and chlorine with respect tooxidation of As is shown in Table 5.

3.7. Biological remediation and other biologicaltreatments

Biological treatment has been demonstrated tobe a useful alternative to conventional treatmentsystems for the removal of toxic metals fromdilute aqueous solution. However, the biopro-cesses for treating toxic effluents must competewith existing methods in terms of efficiency andeconomy. To its advantages, the biotechnological


solution to the problem requires only moderatecapital investment, a low energy input, areenvironmentally safe, do not generate waste inmost cases, and are self-sustaining. It is expectedthat future biotechnological methods of toxicwaste treatment will play a key role as a displace-ment for the existing methods.

Jong and Parry [162] treated arsenic and otheracidic metal (Cu, Zn, Ni, Fe, Al and Mg) andsulfate contaminated waters in a bench-scaleupflow anaerobic packed bed reactor by employ-ing a mixed population of sulfate-reducingbacteria. More than 77.5% of the initial concen-trations of As were removed. Their findings haveagreed well to Simonton et al. [163] work whoreported consistent removal for As and Cr(>6080%) from solution using SRB (Desulfo-vibrio desulfuricans) in columns containing silicasand. It is evident that the action of bacterialsulfate reduction has enhanced the arsenic remo-val rate. Steed et al. [164] had also developed asulfate-reducing biological process to removeheavy metals from acid mine drainage.

Katsoyiannis and Zouboulis [165] studied theremoval of As(III) and As(V) during biologicaliron oxidation. Their results showed that bothforms of arsenic could be efficiently treated forthe concentration range of concentration 50200 mg/L. The bacteria has found to catalyze theoxidation of trivalent arsenic and enhanced theoverall arsenic removal. Other reported worksincluding Mokashi and Paknikar [166] whostudied the arsenic (III) oxidizing processes usingMicrobacterium lacticum in the treatment of arse-nic contaminated groundwater. Papassiopi et al.[167] also reported the use of iron reducingbacteria for the removal of arsenic from con-taminated soils.

Fungal, non-living biomass P. chrysogenum,an industrial waste with trade name Mycan, wasstudied by Loukidou et al. [121] on the removalof arsenates. The mechanism used in their studyis via the biosorption and an effective As(V)removal were obtained in laboratory experiments.

The process was mainly influenced by pH andalso the modification procedures.

3.8. Electrochemical treatment

The electrochemical reduction of inorganicAs(III) and As(V) in aqueous solutions has beenstudied preoperatively with the objective of maxi-mizing the yield of elemental arsenic at theexpense of the highly toxic gas arsine, AsH3. Theelectrochemical removal of As(III) or As(V) fromwastewaters, however, has received little recentstudy.

Twardowski [168] reported a method forremoval of As(III) from mineral acids by electro-chemical reduction to arsenic, which was depo-sited on a three-dimensional carbon cathode,using a divided cell and cathode potentials thatdisfavoured over-reduction to arsine. Bejan andBuunce [169] commented that As(V) was inactiveand could only be removed electrolytically byprior chemical reduction to As (III) in this sys-tem. They used a carbon cathode and IrO2/Tianode to study the electrochemical reduction ofAs(III) and As(V) in acidic and basic solutions.Reduction of As(V) is not efficient, only arsine isremoved. However, efficiency can be improvedby added 5% Pd on alumina as catalyst. Thespeciation of trivalent arsenic in aqueous solutionis principally AsO2! at pH >10, HAsO2 or As2O3at 0 < pH < 10, and AsO+ at pH < 0 [170]. Theelectrochemical reduction is shown as follows:

Cathode: 2H+ + 2e! 6 H2As(III) + 3e! 6 As(0)As(0) +3e! + 3H+ 6 AsH3

Anode: H2O!2e! 6 O2 + 2H+

Bisang et al. [171] also studied the feasibilityof removing the arsenic from acid electro-chemically. They used Cu, Pb, 316L stainlesssteel and graphite as cathodic rotating discs. Thebest results were achieved for copper where the


arsenic deposition takes place in a range ofpotentials without hydrogen evolution. Long-termexperiments with a pilot plant electrochemicalreactor with a three-dimensional cathode alsoshowed that arsenic removal with fractionalconversion per pass of 24% is possible.

3.9. Solar oxidation technique

Solar oxidation in individual units (SORAS)was explored by Garca et al. [172] as alternativetechnology to treat arsenic from the groundwater(Fig. 6). The process is based on photochemicaloxidation of As(III) followed by precipitation orfiltration of As(V) adsorbed on Fe(III)oxides.Their findings show that the underlying chemistryis very complex, and the removal efficiency isaffected by the changes in the chemical matrix, orby changes in the operative conditions. EAWAG,Swiss Federal Institute of Aquatic Science andTechnology, has currently developed SORAS inits laboratory and field tested in the WATSANPartnership Project in Bangladesh [173]. How-ever, more studies are requested before thistechnology is feasible in practical uses of arsenicremoval.

Fig. 6. Basic principle of SORAS with illumination,photochemical formation of the reactive oxidants for theoxidation of As(III) to As(V) and precipitation ofiron(III)(hydr)oxides with adsorbed As(V) [173].

4. Generation and disposal of arsenic residualsafter treatments

As with other production processes, watertreatment systems will produce a residual. Fre-quently, it is the disposal of the treatment arsenicbearing residual and not the treatment technologyitself that is the most difficult issue in practice.Restrictions have been placed on the discharge ofresiduals to water bodies and onto land to preventfurther contamination. This section will focus onresidual generation and disposal of five arsenicremoval systems: anion exchange, activatedalumina absorption, iron/manganese removal,media adsorption, and membrane processes.

4.1. Anion exchanger

A liquid and solid residual may be generatedfrom an anion exchange system. The liquid resi-dual consists of the backwash water, regenerantsolution, and rinse water.These waters constitute1.5 to 10% of the treated water volume dependingon the feed water quality and type of ion ex-change unit used [174]. The spent regenerant maycontain high levels of arsenic or have a corrosivecharacteristic. Spent resin will be produced whenthe resin can no longer be regenerated, or when itbecomes poisoned or contaminated. Spent resinfor disposal may be subject to hazardous wasteregulations depending upon the results of a toxi-city characteristic leaching procedure (TCLP)test. Disposal of these solid wastes (spent resin)are via hazardous waste landfill or return tovendor. In Malaysia, these wastes will be sent toKualiti Alam Sdn Bhd, Bukit Nenas, NegeriSembilan (the sole hazardous waste managementcentre in Malaysia, www.kualitialam.com.my/)and disposed using proper monitor landfill.Liquid effluent will be treated via sanitary seweror ponds/lagoon [175].

4.2. Activated alumina (AA)

A liquid and/or solid residual may be pro-


duced from an AA system depending on the typeof operation. If the system is regenerated, a liquidwaste is produced from the backwash, causticregeneration, neutralization, and rinse steps. Insome instances, a sludge may be generated fromthe regeneration and neutralization streamsbecause some alumina dissolves during theregeneration step and may be precipitated asaluminum hydroxide [176,177] If an aluminumbased sludge is produced because of lowering thepH of the liquid residual, this sludge will containa high amount of arsenic because of its arsenicadsorption characteristics. This sludge and theremaining liquid fraction of the solution willrequire disposal. Because both residuals containarsenic, their disposal may be subject to disposalrequirements.

When the AA has reached the end of its usefullife, the media itself will also become a solidresidual that must be disposed. Because of itshigh arsenic removal capacity, an activated alu-mina system may be operated on a media throw-away basis rather than a media regeneration basis.When operated on a throw-away basis, theexhausted AA media will be the principal residualproduced. This media has the potential of beingclassified as a hazardous waste because of itshigh arsenic content. A TCLP test is necessary,therefore, to determine its classification andultimate disposal restrictions. Because the AAmedia will filter out particulate material in thesource water, the media bed will occasionallyrequire backwashing. This backwash water willlikely contain some arsenic attached to either theparticulate material or the very fine AA materialthat is removed during backwashing. Conse-quently, the disposal of the backwash water mayalso be subject to the disposal requirements [175].

4.3. Adsorption

In this method, contaminated water is passedthrough a bed of the specially developed media,where arsenic is adsorbed and removed from the

water. Two general types of residuals are poten-tially generated from media adsorption: spentmedia and regeneration solutions. Spent mediawill be generated from systems that use the mediaon a one-time throw-away basis, or from systemswhere the media has become exhausted and canno longer be regenerated, or is no longer effec-tive. In some cases, depending on manufacturerpolicy, spent media may be sent back to thevendor for reactivation, recovery, or disposal.

No details were provided for regeneration. Itis generally assumed that the same steps as forion exchange will be utilized: backwash, regene-ration, and rinse. Each of these steps will generatean aqueous residual which will likely be com-bined. Some of the new adsorption media havesuch large arsenic removal capacities that peri-odic backwashing (with regeneration) is requiredto remove the particulate material that is filteredout during its treatment operation. This backwashwater will likely contain some arsenic that isattached to the particulate material or any veryfine adsorption media that is removed by thebackwashing process. The waste stream is aresidual that may be disposed of immediately atthe time of backwashing or it may be held anddisposed with the regeneration waste water.Depending on the concentration of arsenic in theinfluent and other factors, the disposal of theregeneration waste and the backwash water maybe subject to the disposal requirements [175].

4.4. Iron/manganese removal methodsIron/manganese removal processes, both the

oxidation/filtration and the potassium perman-ganate greensand techniques, produce a liquidresidual from the filter backwashing step (Fig. 7).Occasionally, the filter media or greensand needsto be replaced and this material also becomes aresidual product that must be disposed. Similar tothe backwash and regenerant solution from theion exchange and activated alumina processes,the filter backwash water will contain arsenic, the


Fig. 7. Schematic of oxidation-filtration Fe/Mn removal process [175].

Table 6Residual generation and disposal for the various arsenic treatment methods [175]

Treatment method for arsenic Form of residual Residual generation Disposal

Ion exchange Liquid Regeneration streamsSpent backwashSpent regenerantSpent rinse stream

Sanitary sewerDischargeEvaporation ponds/lagoon

Solid Spent resins LandfillHazardous waste landfillReturn to vendor

Activated alumina Liquid Regeneration streams Spent backwash Spent regenerant (caustic) Spent neutralization (acid)Spent rinseLiquid filtrate (when brine streams are precipitated)

Sanitary sewerDirect dischargeEvaporation ponds/lagoon

Solid Spent alumina Sludge (when brine streams are precipitated)

LandfillHazardous waste landfillLand application

Adsorption Liquid Regeneration streams Spent backwash Spent regenerantSpent rinse stream

Sanitary sewerDirect dischargeEvaporation ponds/lagoon

Iron and manganeseremoval processes

Liquid Filter backwash Direct dischargeSanitary sewerEvaporation ponds/lagoons

Solid Sludge (if separated from backwash water)Spent media

Sanitary sewerLand applicationLandfillLandfillHazardous waste landfill

Membrane processes Liquid Brine (reject and backwash streams)

Direct dischargeSanitary sewerDeep well injectionEvaporation ponds/lagoon


concentration dependent upon the amount ofarsenic removed and the quantity of backwashwater. Although the liquid fraction of the back-wash water will contain some soluble arsenic,most of the arsenic will be associated with theiron/manganese solids. Depending upon theirarsenic concentration, the disposal of the back-wash water residual and the spent solid mediaresidual may be subject to the disposal require-ments [175].

4.5. Membrane processes

All membrane processes produce a rejectwaste product containing the materials, includingarsenic, rejected by the membrane. The rejectwater is generally high in total dissolved solids[174]. Depending on the concentration of thearsenic and other contaminants in the rejectwater, the disposal of this waste may be subject tothe disposal requirements.

Each treatment technology in arsenic removaldescribed above differs in residual production andresidual management options. Table 6 presents asummary of these five unit processes, the type ofresidual produced, and a list of possible disposalmethods for the residuals.

5. Conclusion

To remove arsenic from wastewaters, the mostcommonly used technologies are adsorption ontoactivated alumina, and precipitation or adsorptionby metals oxides, predominantly Fe(III) andmembranes. These technologies for removal ofarsenic from wastewaters are most suited to deal-ing with relatively low concentrations of arsenic,i.e. the low g/l level. However, the technique ofprecipitation, generally using Fe (III) or limesoftening is suited to higher concentrations,normally at the low mg/l levels. Adsorption is amethod that has been an important method usedin arsenic removal. Most studies are focused on

the type of adsorbent mediums and the economicsof their regeneration. Membrane technology,especially nanofiltration, becomes a promisingmethod in arsenic removal and is also widelyconsidered as the methods that can be used tomeet regulations for lowered arsenic concen-trations in drinking water. Other alternativemethods also studied for their feasibility inreplacing the current available methods. Futureneeds on arsenic removal technology should takeinto considerations of reducing the treatment cost,simplifying the operational complexity of thetechnology and disposal of arsenic bearingtreatment residual.

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