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Available online at www.sciencedirect.com REACTIVE

Reactive & Functional Polymers 67 (2007) 1599–1611

www.elsevier.com/locate/react

&FUNCTIONALPOLYMERS

Use of ArsenXnp, a hybrid anion exchanger, for arsenicremoval in remote villages in the Indian subcontinent

Sudipta Sarkar a, Lee M. Blaney a, Anirban Gupta b, Debabrata Ghosh b,Arup K. SenGupta a,*

a Department of Civil and Environmental Engineering, Lehigh University, Bethlehem, PA 18015, USAb Environmental Engineering Cell, Civil Engineering Department, Bengal Engineering and Science University, Howrah, India

Available online 1 August 2007

Abstract

Many of the arsenic removal units operating in remote villages of West Bengal, India now use a hybrid anion exchanger(HAIX) which are essentially spherical anion exchange resin beads containing dispersed nanoparticles of hydrated ferricoxide (HFO). HAIX, now commercially available as ArsenXnp, offers a very high selectivity for sorption of oxyanions ofarsenic due to the Donnan membrane effect. The sorption columns used in the field for removal of arsenic are either singlecolumn or split-column design. The sorption columns allow flow of atmospheric oxygen, thereby promoting oxidation ofdissolved Fe(II) species of arsenic-contaminated raw water to insoluble Fe(III) oxides or HFO particulates. Apart from theusual role played by the sorbents like ArsenXnp or activated alumina towards arsenic removal, HFO particulates also aidin the treatment process. Each unit is attached to a hand-pump driven well and capable of providing arsenic-safe water tothree hundred (300) households or approximately one thousand villagers. No chemical addition, pH adjustment or elec-tricity is required to run these units. On average, every unit runs for more than 20,000 bed volumes before a breakthroughof 50 lg/L of arsenic, the maximum contaminant level in drinking water in India, is reached. In addition to arsenicremoval, significant iron removal is also achieved throughout the run. Upon exhaustion, the media is withdrawn and takento a central regeneration facility where 2% NaCl and 2% NaOH solution are used for regeneration. Subsequently, theregenerated resin is reloaded into the well-head sorption column. Following regeneration, the spent solutions, containinghigh arsenic concentration, are transformed into solids residuals and contained in a way to avoid any significant arsenicleaching. Laboratory investigations confirmed that the regenerated ArsenXnp is amenable to reuse for multiple cycles with-out any significant loss in capacity.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Arsenic; Hybrid ion exchanger; Ion exchange; ArsenXnp; Groundwater

1381-5148/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.reactfunctpolym.2007.07.047

* Corresponding author. Present address: Fritz Laboratory, 13E Packer Avenue, Bethlehem, PA 18015, USA; Tel.: +1 610 7583534; fax: +1 610 758 6405.

E-mail address: [email protected] (A.K. SenGupta).

1. Introduction

Arsenic present in drinking water drawn fromunderground sources is the cause of wide-spreadarsenic poisoning affecting nearly 100 million peopleliving in Bangladesh and West Bengal, a neighboring

.

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Indian state [1–4]. While the maximum contaminantlevel (MCL) of arsenic in drinking water is 50 lg/L[5,6] in India, arsenic concentrations in this regionwell exceed the MCL. Health effects related toarsenic ingestion through drinking water take a longtime before becoming fatal and life-threatening [7].Average annual precipitation in this geographiclocation is significantly high, often exceeding1500 mm/year. But poor sanitation practices whichprevail in this area have contaminated surface watersleading to a potential risk of water borne diseases ifused as drinking water without appropriate treat-ment. On the other hand, relative abundance andease of finding bacteriologically safe groundwatersources promoted the wide-spread use of wells withhand pumps as drinking water sources. Thereremain thousands of villages where arsenic-lacedground water is the only viable source of drinkingwater.

Several treatment technologies and equipmenthave been developed for removal of arsenic fromwater. It is well known that hydrated oxides of poly-valent metals like Fe(III), Al(III), Ti(IV) and Zr(IV)exhibit ligand sorption properties by forming inner-sphere complexes [8–13]. A non-regenerable adsorp-tion media, granulated ferric hydroxide (GFH) hasbeen widely used in many places including WestBengal, India [14]. It has also been reported thatthe above-mentioned metal oxides, when dispersedwithin a polymeric host material, offer tunablebehaviors for sorption of a wide variety of anionicligands and transition metal cations [15–18]. Onesuch hybrid sorbent, produced by dispersinghydrated ferric oxide (HFO) nanoparticles inside apolymeric anion exchanger host material, exhibitshigh affinity for removal of arsenic from naturalwaters due to the Donnan membrane effect exertedby the host material [18–20]. The hybrid anionexchanger (HAIX) is now commercially availableas ArsenXnp from SolmeteX Co. in Northborough,MA and Purolite Co. in Philadelphia, PA; however,no endorsement is implied. Earlier investigationsshowed that the chelating polymers with nitrogendonor atoms, when loaded with copper(II), are veryselective to inorganic arsenic species and also arereusable [21–23]. However, high price of the parentchelating polymer was a major obstacle towardwider applications related to water and wastewatertreatment.

Since 1997, Bengal Engineering and Science Uni-versity, Howrah, India and Lehigh University, USAhave collaborated to develop a sustainable solution

to combat the arsenic problem in West Bengal,India. Under this initiative, about 160 well-headarsenic removal systems have been installed. Theseunits are community based and serve about 250–300 families; additionally, the units require no elec-tricity, chemical addition or pH adjustments. Theadsorbent media used commonly is activated alu-mina. Characteristics and performance of theseunits have been previously reported [24]. Since2004, ArsenXnp media, along with activated alu-mina has been utilized in the units. The primaryobjective of this article is to present the performanceof ArsenXnp for arsenic removal over a long periodof run in the field, regenerability of the media, andelucidation of arsenic removal mechanism and con-tainment strategies of arsenic removed.

2. Experimental

2.1. Well-head treatment units

The main component of the well-head treatmentunit is an adsorption column (diameter 35 cm,height 2 m), which is a gravity-fed system operatingin downflow mode. Apart from the adsorption col-umn, there is a coarse-sand filter to contain thebackwash waste water from the column, which con-tains arsenic-laden precipitates of ferric hydroxideor hydrated ferric oxide (HFO). The adsorption col-umn mounted on top of a hand-pump driven well isa stainless steel (SS304) cylindrical tank with twodistinct functional regions. At the top of the tank,there are atmospheric vent connections to allowpassage of atmospheric oxygen. The inlet water issprayed in and is further divided in fine dropletsby means of a splash distributor installed at thetop of the tank. Sufficient volume is kept at thetop of the tank to facilitate longer residence timeto allow oxidation of dissolved Fe(II) species toinsoluble Fe(III) species before the well water entersthe second region, which contains fixed-bed of sor-bent media supported by graded gravels. Ulti-mately, arsenic-safe treated water is collected atthe bottom of the unit. The sorbent used isArsenXnp. The necessary constructional and opera-tional features of the adsorption column are sche-matically indicated in Fig. 1a. The amount ofsorbent employed in each column is approximately100 kg. The design flow rate through the columnis 8–10 L/minute. The column is routinely back-washed every morning for about 10–15 min in orderto drive out precipitated HFO particles and to

Fig. 1. (a) Schematic detail and operational mechanism of the well-head unit and (b) details of the coarse-sand filter for entrapment ofwaste backwash containing HFO particles (all dimensions in mm).

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maintain a desirable flow rate. Fig. 1b provides per-tinent details of the coarse-sand filter used to trapHFO particles from the waste backwash water.

Another type of design later evolved where thesingle adsorption column is split in two separate col-umns in order to optimize on the use of sorbent like

Fig. 2. Schematic detail of construction and opera

HAIX and to take better advantage of HFO precip-itates. Fig. 2 provides schematic details of the split-column design. The uppermost column containscomponents necessary for oxidation of dissolvediron similar to the single-column unit; however, sor-bent media is only 50 kg of activated alumina. The

tion of a split-column unit used in the field.

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partially treated water then enters the second col-umn containing 50 kg ArsenXnp, which basicallyacts as a polishing unit. Finally, treated water is col-lected at the bottom of the second column. Both col-umns have provisions for backwashing. However,the second column does not require backwashbecause iron oxidation and precipitation onlyoccurs in the first column. At the effluent port ofeach unit, a flow totalizer (mechanical type) is pro-vided to record total volume of water treated.

2.2. Preparation and characteristics of HAIX

SolmeteX, Inc. manufactures ArsenXnp using theprocedure developed at Lehigh University [25]. Dis-persing HFO nanoparticles within an anion exchan-ger is scientifically challenging because the ferric ion(Fe3+) and quaternary ammonium group (R4N+) inanion exchange are both positively charged. Fig. 3illustrates the multi-step process protocol forArsenXnp synthesis. The parent anion exchanger ismacroporous strong base type and has polystyrenematrix with quaternary ammonium functionalgroups; the capacity is 0.8 meq/g of resin. Fig. 4shows a photograph of the beads and TEM imageof the interior of the bead. HFO loading on theanion exchanger was found to be 150 mg Fe/g.The hybrid sorbent with size range of 0.5–0.7 mm

Fig. 3. Synthesis of hybrid anion exchanger or A

is mechanically strong, durable and suitable foruse in fixed-bed columns.

2.3. Sample preparation and analysis

For analysis of As(V) and As(III), samples col-lected at site are adjusted to pH 4.0 and are imme-diately separated using a strong-base anionexchange resin mini-column in accordance with atechnique developed earlier [26,27]. Total arsenic isdetermined from the original sample, As(III) fromthe sample collected at the exit of the anionexchange column and As(V) by difference. The tech-nique is validated using samples of known As(III)and As(V) concentrations.

Bengal Engineering and Science University ana-lyzes arsenic using a automatic flow injection atomicabsorption spectrophotometer (Chemito, India)with hydride vapor generation accessory. At LehighUniversity, arsenic is analyzed using an atomicabsorption spectrophotometer with graphite fur-nace accessory (Perkin–Elmer model SIMAA6000). Samples for analysis of iron are preservedat pH < 2 through addition of 8 M HNO3.

Dissolved oxygen, hardness, alkalinity, silica andphosphate analyses are carried out in accordancewith the procedures available in Standard Methods[28].

rsenXnp from parent anion exchange resin.

Fig. 4. (a) Photograph of ArsenXnp and (b) TEM image of the interior of the bead.

S. Sarkar et al. / Reactive & Functional Polymers 67 (2007) 1599–1611 1603

All laboratory isotherms and column tests wereconducted with background concentrations of com-monly occurring electrolytes. Fixed bed columnruns utilized ELDEX fraction collectors. Superficialliquid velocity and empty bed contact times (EBCT)were recorded for every column run. Isotherm testswere carried out in plastic bottles in a gyratoryshaker.

An extended toxicity characteristic leaching pro-cedure (ETCLP) was also performed with the driedsludge (treatment residual) at different pH towardsdetermination of leaching potential [29].

2.4. Regeneration

Upon exhaustion of the adsorption column,media from each unit is replaced by fresh or regen-erated media. The exhausted ArsenXnp and acti-vated alumina is taken to a central regenerationfacility where the media is regenerated inside a man-ually operated stainless steel batch reactor that canbe manually rotated about its horizontal axis. Fig. 5is a sketch of the batch reactor used forregeneration.

For regeneration of ArsenXnp, a solution con-taining 2% NaOH and 2% NaCl is utilized; theexhausted ArsenXnp is reacted with two bed vol-umes of regenerant solution in the batch reactorfor 45 min. The process is repeated once with a freshregenerant solution. During regeneration, pHremains near 12.0; spent alkali is collected. After athorough rinse with well water, the media is sub-jected to two bed volumes of dilute HCl solutionto neutralize the media so that resultant solutionpH is 5.5; subsequently, the spent acid is collected.The media is then rinsed with well water.

The general procedure for regeneration ofexhausted activated alumina is similar except thatNaCl is not used in the regenerant. The exhaustedmedia is air-dried and kept in a safe place for reuse.At the end of the regeneration, spent acid, alkali andrinse water are mixed together and pH is adjusted to6.5 by adding 10% hydrochloric acid. Thick brownslurry immediately forms and settles overnightbefore being disposed of on top of a coarse-sand fil-ter. Arsenic-laden solids and HFO particles areintercepted and retained at the top of the filter.The entire regeneration, including the spent regener-ant treatment, is completed in 5 h.

3. Results

3.1. Isotherms for activated alumina and ArsenXnp

Figs. 6 and 7 show the As(III) and As(V) adsorp-tion isotherms onto ArsenXnp and activated alu-mina, respectively. It may be noted that As(III)and As(V) isotherms for ArsenXnp are comparable,but activated alumina prefers As(V) well overAs(III).

3.2. Performance of the single and split-column unit

Fig. 8 shows performance of a single-column unitcontaining ArsenXnp located at Nabarun Sangha,Kankpul, Ashoknagar in North 24 Parganas districtof West Bengal. For an average inlet concentrationof 85 lg/L, the unit ran for almost 29,000 bed vol-umes before breakthrough of 50 lg/L. The break-through curve is gradual in nature. Arsenicspeciation in the raw water demonstrated an arse-nate, or As(V), to arsenite, or As(III), ratio of

Fig. 5. Sketch of the batch reactor used at central regeneration facility.

Fig. 6. Isotherm of ArsenXnp and activated alumina (AA) for adsorption of As(III) species.

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60:40. Silica and phosphate concentration in theinfluent water were 19 and 3 mg/L, respectively.The dissolved iron concentration was 3.5 mg/L.After passage of 5800 bed volumes of water, the unitwas connected to an adjacent well due to problemsin the existing well. With lower dissolved iron andphosphate concentration of 2.5 mg/L and 0.5 mg/L, respectively, however, no significant impact was

observed in the breakthrough curve. There was anearly breakthrough of silica at less than 1500 bedvolumes; while phosphate breakthrough occurredat 3000 bed volumes. Approximately 164.9 mg oftotal arsenic was removed by the ArsenXnp column.

Fig. 9 represents the arsenic history for a split-column unit located at Binimoypara, Ashoknagarin the North 24 Parganas district of West Bengal,

Fig. 7. Isotherm of ArsenXnp and activated alumina (AA) for adsorption of As(V) species.

Fig. 8. Arsenic removal performance of a single-column unit at Nabarun Sangha, Ashoknagar, West Bengal. The closed squares representarsenic concentration in raw water whereas the closed triangles represent arsenic concentration in treated water.

S. Sarkar et al. / Reactive & Functional Polymers 67 (2007) 1599–1611 1605

India. For an average inlet arsenic concentration of160 lg/L, the column ran for approximately 22,000bed volumes. The ratio of As(V) to As(III) in rawwater was 60:40. Analysis of the breakthroughcurves demonstrates that approximately 228.8 mg

of total arsenic was removed in the first columnloaded with activated alumina; the second columnwhich is loaded with ArsenXnp, removed about54.4 mg of total arsenic. It may be noted thatalthough activated alumina showed a poorer

Fig. 9. Arsenic removal performance of the split-column unit at Binimoypara, Ashoknagar, West Bengal. The closed squares, closedtriangles and open triangles denote raw water, effluent from first column and final treated water, respectively.

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adsorption capacity compared to ArsenXnp forremoval of both As(V) as well as As(III), (Figs. 6and 7), activated alumina column removed muchmore arsenic compared to ArsenXnp column.

3.3. Iron removal

Figs. 10 and 11 represent the iron histories of thesingle column as well as the split-column unit,respectively. It is observed that in both cases, thereis a significant removal of dissolved iron that wasoriginally present in the raw water. Also, the con-centration of iron in the treated effluent does nothave any dependence on the number of bed volumespassed through the column. This evidence pointsout that the removal mechanism is not adsorptivebut typically an oxidative precipitation followedby filtration. Iron removal in the arsenic removalunits is possible due to its unique construction fea-tures that allow oxidation of Fe(II) species to aninsoluble Fe(III) species. It may also be noticed thatfor the split-column unit, the iron removal tookplace in the first column only. This observation alsosignifies the advantage of the split-column design;users need to backwash only the first column inorder to wash out the HFO particles. The totalamount of iron removed by the unit at NabarunSangha was approximately 5820 g. The first column

of the split-column unit at Binimoypara removedabout 4600 g of total iron.

3.4. Regeneration of HAIX and safe transformationof treatment residues

Regeneration of the exhausted media was carriedout at a central regeneration facility following theprocedure indicated earlier. Table 1 shows arsenicand iron concentrations in the spent solutions(treatment residual) from the regeneration of theexhausted ArsenXnp of the single-column unit atNabarun Sangha. It may be noted that the spentregenerant along with the acid and water rinses con-tain high concentrations of arsenic. In an effort todetoxify the spent solutions, all the spent regener-ants are mixed together, the pH is lowered to 6.5–7.0 producing thick brown precipitates of ferrichydroxide. The particulates of ferric hydroxideadsorb arsenic from the bulk solution leaving thebulk phase with a fairly low concentration of arsenic(Table 1). A mass balance on the regeneration datademonstrates that the dry weight of the precipitatedmass (sludge) should not be more than 1 kg. Thebulk water along with the sludge is disposed ontop of a coarse-sand filter which is similar to theone described in Fig. 1b but larger in size. The topof the coarse-sand filter is open to atmosphere.

Fig. 10. Iron removal by single-column unit at Nabarun Sangha, Ashoknagar, West Bengal. Closed squares denote iron concentration ininlet water whereas closed triangles denote the same in the treated water.

Fig. 11. Iron removal by the split-column unit at Binimoypara, Ashoknagar, West Bengal. Closed squares, closed triangles and opentriangles denote raw water, effluent from first column and final treated water, respectively.

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Fig. 12 shows the leaching potential of a simi-lar sludge (obtained from regeneration of acti-vated alumina) at different pH. The resultsdemonstrate that the arsenic concentration in theleachate tends to be minimum at a pH 5.5; arsenic

was present as As(V). Earlier laboratory studiessuggest that ArsenXnp can adsorb As(V) speciesover many cycles of sorption and desorption with-out significant capacity loss. Fig. 13 provides suchdata.

Fig. 12. Leaching potential of the treatment residue as determined by ETCLP test.

Table 1Volumes and compositions of individual regenerant streams for regeneration of exhausted media used at Nabarun Sangha, Ashoknagar, N24 Parganas, West Bengal, India

Description Volume (L) pH Arsenic (lg/L) Total iron (mg/L)

Spent caustic 1st batch 140 12.5 39600 2500Spent caustic 2nd batch 140 12.5 11200 245Acid rinse 180 5.5 320 2.86Treated wastewater 460 7.1 30 0.96

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4. Discussion

4.1. Arsenic removal: design of well-head units and

role of dissolved iron

The uppermost part of the unit ensures near-complete oxidation of dissolved iron to hydratedferric hydroxide or HFO by oxygen as shown below:

4Fe2þ þO2 þ 10H2O! 4FeðOHÞ3ðsÞ þ 8Hþ

� ðDG0Reac ¼ �18kJ=moleÞ

ð3Þ

The standard state free energy change for the abovereaction is highly negative implying that the forwardreaction is thermodynamically favorable. Hydrogenions generated by the precipitation reaction are neu-tralized instantaneously by alkalinity (HCO�3 Þ pres-ent in groundwater. As a result, no significant pHchange has been observed at any site regardless of

dissolved iron content. X-ray diffraction (XRD)analysis confirmed that precipitated HFO particlesare present in the amorphous state and no crystal-line iron oxide (e.g., goethite, hematite) was formedeven after several weeks.

In an activated alumina column of similar design,precipitates of HFO particles played a vital role forremoval of arsenic, especially As(III) [24,30]. As(III)oxidation is thermodynamically possible but previ-ous field observation has indicated that there is min-imal conversion of As(III) to As(V) within the bed[24]. So, from a mechanistic view point, the role offreshly precipitated HFO is significant. Fig. 14depicts arsenic removal mechanisms that are opera-tive within the adsorption column.

The exceptionally high removal capacity shownby the activated alumina column (Fig. 9) as com-pared to ArsenXnp is thus attributable to adsorptioneffort provided by freshly precipitated HFO in coor-dination of activated alumina. It has been reported

Fig. 13. Performance of ArsenXnp for multiple cycles of sorption–desorption in the laboratory.

Fig. 14. An illustration depicting interplay of different variables for simultaneous removal of As(III) and As(V) in the adsorption column.

S. Sarkar et al. / Reactive & Functional Polymers 67 (2007) 1599–1611 1609

that high concentration of dissolved iron in waterallow formation of HFO particulates towards suc-cessful household arsenic treatment in Vietnam[31,32].

4.2. Residue management

Managing and containing arsenic-laden wasteproducts is almost as important as removing arsenicfrom drinking water. Local environmental laws/

guidelines with regard to the safe disposal ofarsenic-containing treatment residuals do not existor are not enforceable. However, to avoid futurehazard, proper management of treatment residualsis considered an important part of the overall treat-ment scheme.

Regeneration of the media is the first steptowards volume reduction of the treatment residu-als. If there was no regeneration, every well-headunit would produce 100 kg of disposable sorbent

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media at the end of each cycle, posing a sizeableobstacle to safe waste management. Followingregeneration and containment of arsenic into abouta kilogram of solid sludge, the amount of treatmentresidue is reduced by approximately 100 times. Thesludge is contained and stored at the top of a coarse-sand filter which is deliberately designed to maintainatmospheric conditions.

The oxidizing environment inside of coarse-sandfilters helps to keep iron in the form of Fe(III) andarsenic in As(V) form. Fig. 15 presents a pe–pH orpredominance diagram for arsenic. During theregeneration procedure, all the solutions are nearlysaturated with atmospheric oxygen. The followinghalf reaction and the resulting pe value tend todetermine the redox environment for them [33]:

1=4O2 þHþ þ e� ¼ 1=2H2O pe0 ¼ 20:79 ð4Þpe ¼ pe0 þ 1=4 log PO2 � pH ð5Þ

where, PO2 = partial pressure of oxygen = 0.21 atmfor atmospheric oxygen

The estimated boundary of pe–pH conditions forboth spent alkali regenerant and the precipitatedsludge are marked on the pe–pH diagram. It maybe noted that As(V) is the dominating species inboth situations. Experimental observations supportthis fact. Since any reduction of HFO particles,

Fig. 15. pe–pH diagram of arsenic confirming the predominanceof As(V) in the precipitated sludge after the spent regeneranttreatment.

which is the predominant constituent of the sludge,to Fe(II) will result in an enhanced leaching ofarsenic, the top of the coarse-sand filter is deliber-ately kept open to atmosphere through provisionof vents. In this region, the presence of Fe(III)and As(V) is thermodynamically favorable. Hence,there will not be any significant leaching of arsenicas long as an oxidizing condition prevails, and thepH of the water passing through the storage cham-ber is in the range 5 –10. Experimental observationsof the leaching study carried out in such conditionsindicated a very minimal leaching of arsenic, in thetune of 30–100 lg/L.

On the other hand, if the sludge is kept underreducing environment, such as in sanitary landfills,it is thermodynamically possible that arsenic andiron will be reduced to As(III) and Fe(II), respec-tively, causing enhanced leaching. TCLP (ToxicityCharacteristic Leaching Procedure) may indicateminimal leaching of arsenic from the sludge, as itis performed under oxidizing environment [34].However, under reducing condition, such as thatinside landfills there will be enhanced leaching ofarsenic [35,36]. Also, there is evidence of researchfindings that TCLP underestimates leaching ofarsenic as the protocol essentially proposes to carryout tests with a headspace of air in the bottles [37].

5. Conclusions

ArsenXnp is the first polymer-based commerciallyavailable arsenic-selective sorbent. The field perfor-mances of well-head arsenic removal units usingArsenXnp demonstrate that such units can effec-tively produce arsenic-safe water for more than20,000 bed volumes. The units do not require anychemical addition or electricity. These communitybased units are run and maintained by the villagers.At the end of each run, the exhausted sorbent isregenerated at a central regeneration facility; afterregeneration, the treatment residue is contained asa solid sludge. Furthermore, water treatment resid-uals amount to approximately 1 kg, about 100 timeslower than the exhausted sorbent which if disposedof in a landfill would have leached significant con-centration of arsenic back to the environment. Thesolid sludge is disposed on top of a sand chamber,which under atmospheric conditions, preventsarsenic leaching. Therefore the treatment technol-ogy and the residue management offer a sustainablesolution for the problem of arsenic in drinkingwater. The technology can be replicated in other

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developing nations including Mexico and Vietnamwhere arsenic in groundwater poses severe threatto human health.

Acknowledgements

Partial financial assistance from private donorslike Hilton Foundation and Rotary Internationalthrough Water For People is acknowledged. Also,the authors would like to thank SolmeteX, Inc.for providing ArsenXnp for the units installed inthe villages. The authors like to thank Mr. AlokPal, Mr. Dilip Ghosh and Mr. Morshed Alam fortheir assistance in field and laboratory work.

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