Prolonged testing of metal mobility in mining-impacted soils amended with phosphate fertilisers

Prolonged Testing of Metal Mobility in Mining-ImpactedSoils Amended with Phosphate Fertilisers

Niels C. Munksgaard & Bernd G. Lottermoser &

Kevin Blake

Received: 31 May 2011 /Accepted: 1 November 2011 /Published online: 23 November 2011# Springer Science+Business Media B.V. 2011

Abstract The aim of the study was to determinewhether the application of superphosphate fertiliser tosoils contaminated with mine wastes can inhibit metaland metalloid mobility (Cu, Pb, Zn, Cd, Fe, Mn, As, Sb)in the long term. Contaminated soils contained sulfide-and sulfate-rich waste materials from the Broken Hilland Mt Isa mining centres. Results of long-term (10months) column experiments demonstrate that fertiliseramendment had highly variable effects on the degree ofmetal and metalloid mobilisation and capture. Rapidrelease of metals from a sulfate-rich soil showed thatphosphate amendment was ineffective in stabilisinghighly soluble metal-bearing phases. In a sulfide-richsoil with abundant organic matter, complexing of metalswith soluble organic acids led to pronounced metal(mainly Cd, Cu and Zn) release from fertiliser-amendedsoils. The abundance of pyrite, as well as the addition offertiliser, caused persistent acid production over time,

which prevented the formation of insoluble metalphosphate phases and instead fostered an increasedmobility of both metals and metalloids (As, Cd, Cu, Sb,Zn). By contrast, fertiliser application to a sulfide-richsoil with low organic carbon content and a sufficientacid buffering capacity to maintain near-neutral pHresulted in the immobilisation of Pb in the form of newlyprecipitated Pb phosphate phases. Thus, phosphatestabilisation was ineffective in suppressing metal andmetalloid mobility from soils that were rich in organicmatter, contained abundant pyrite and had a low acidbuffering capacity. Phosphate stabilisation appears to bemore effective for the in situ treatment of sulfide-richsoils that are distinctly enriched in Pb and containinsignificant concentrations of organic matter and othermetals and metalloids.

Keywords Mine waste .Metals . Fertiliser .

Immobilisation . Remediation

1 Introduction

Metal contamination of soils has received considerableattention in recent decades and several remediationtechnologies have been pursued, including chemical andphysical extractions and in situ stabilisation techniques.In situ treatment of metal-contaminated soil, such asphosphate stabilisation, has become an attractive reme-diation option compared to conventional engineering-based solutions because in situ treatment would not

Water Air Soil Pollut (2012) 223:2237–2255DOI 10.1007/s11270-011-1019-y

N. C. Munksgaard (*)School of Earth and Environmental Sciences,James Cook University,PO Box 6811, Cairns, Qld 4870, Australiae-mail: [email protected]

B. G. LottermoserSchool of Earth Sciences, University of Tasmania,Private Bag 79,Hobart, Tasmania 7001, Australia

K. BlakeAdvanced Analytical Centre, James Cook University,Townsville, Qld 4811, Australia

necessitate the physical removal of the soil from itsoriginal site. Phosphate stabilisation is based on theaddition of a phosphate source (e.g. phosphorite rock,bone meal, phosphate fertiliser) to metal-rich waste orsoil to induce the formation of relatively insoluble metalorthophosphates (Ma et al. 1995; Cotter-Howells andCaporn 1996; Laperche et al. 1996; 1997; Zhang et al.1997, 1998; Ma and Rao 1999).

Extensive research on phosphate treatment of Pb-contaminated soils and wastes has led to the broadacceptance amongst researchers of phosphate asstabilising agent for Pb-contaminated soils and wastematerials (e.g. Zhang and Ryan 1998; Hettiarachchi etal. 2001; Ryan et al. 2001; Cao et al. 2002; Chen et al.2003; Melamed et al. 2003; Scheckel and Ryan 2004;Lin et al. 2005; Wilson et al. 2006; Yoon et al. 2007;Miretzky and Fernandez-Cirelli 2008; Bosso et al.2008; Park et al. 2011). However, to our knowledge,no practical application of phosphate stabilisation in themining industry or by statutory authorities, other thanrelatively small field trials, has been reported. Success ofapplying phosphate to metal-contaminated soils remainsquestionable because: (a) many contaminated soils donot only contain a singular metal but a complexity ofother metals or metalloids; (b) the use of bone mealpromotes organic complexing of Pb and Cu and theirleaching from soils (e.g. Martinez et al. 2004); (c) theapplication of acid-generating phosphate com-pounds can lead to the acidification of soils andpronounced Mn, Cu and Zn mobility (Munksgaardand Lottermoser 2011); (d) the higher-solubilityproducts of Zn and Cu phosphates compared to Pbphosphates make phosphate immobilisation of Zn andCu less effective than Pb (Ma et al. 1995; Cao et al.2003); and (e) due to the necessity to maintain solublephosphate concentrations well in excess of the stoichio-metric requirements of metal orthophosphates, dis-solved phosphate may be transferred to deeper soillevels (Cao et al. 2002; Chen et al. 2003). Moreover,there is little knowledge on whether metals are capturedat the site of treatment or are mobilised and transportedto lower soil levels, possibly contaminatingwaters in theunsaturated and saturated zone.

This project was conducted to explore the mobility ofmetals (Cu, Pb, Zn, Cd, Fe, Mn) and metalloids (As, Sb)in phosphate-amended ‘anthrosols’ (waste-contaminatedland) from Broken Hill and Mt Isa, where miningoperations have occurred for many decades and localsoils are well known for their elevated metal

concentrations (Gulson et al. 1994; Taylor et al.2010). In this study, material characterisation and10-month column leach experiments were completedto evaluate: (a) the siting of metals and metalloids inphosphate-amended top soils and (b) the leaching ofmetals, metalloids, phosphate and sulfate into lowersoil levels during a 10-month laboratory trial. Thus,this research adds to our understanding of phosphatetreatment of contaminated ground and contributes toevaluating phosphate stabilisation as a successful andsustainable treatment method.

2 Materials and Methods

2.1 Materials

The waste-impacted ‘anthrosols’ used in this study werecollected from the major mining centres of Mount Isaand Broken Hill, Australia (Fig. 1). Ore extracted fromthe Broken Hill ore bodies consists of major galenaand sphalerite (Plimer 1984). Mount Isa Pb–Zn–Agore contains mainly galena, sphalerite, pyrite andpyrrhotite, whereas the primary Cu ore minerals arechalcopyrite, pyrite and pyrrhotite (Forrestal 1990;Perkins 1990). Soils in both mining centres arenaturally mineralised and have been contaminatedwith metals and metalloids as a result of mine wastedumping, atmospheric fallout from smelter emissionsand dust deposition originating from ore stockpiles,tailings storage facilities and exposed mine workings.Consequently, Mount Isa soils are enriched in Cu, Pband Zn (plus minor As and Cd), whereas Pb and Zn(plus minor As, Cd and Sb) dominate in Broken Hillsoils (Gulson et al. 1994; Taylor et al. 2010).

A soil sample (MS1) was obtained from Mount Isaimmediately east of the smelting works at UTM 54K0342280 E, 7707540 N. At Broken Hill, two soilsamples were collected just south of the ConsolidatedBroken Hill mining lease at UTM 54J 0544358 E,6462872 N (sample BH1) and at UTM 54J 0544556 E6463097 N (sample BH2). Each sample consisted of ∼30subsamples (depth 0–0.05 m; ∼3 kg), taken from a 4-m2

area to improve site representativeness and compos-ited to yield bulk soil samples (∼100 kg each).Chemical and mineralogical data for the Broken Hillsoil samples have already been documented byMunksgaard and Lottermoser (2011), whereas MountIsa soil data are presented in this work. Commercial-

2238 Water Air Soil Pollut (2012) 223:2237–2255

grade superphosphate (‘SuPer’, Incitec Pivot Ltd), apartially soluble granular fertiliser consisting of Ca-orthophosphate and anhydrite (CaSO4), was used asphosphate amendment in the soil leaching tests.

2.2 Material Characterisation

Soil samples were air-dried, sieved to <2 mm andhomogenised. Soil pH was determined in 1:5 soil/watersuspension and net acid generation (NAG) pHmeasure-ments were performed after complete oxidation byhydrogen peroxide, following the procedure by Morinand Hutt (1997). Such NAG pH measurements providea preliminary evaluation of the soils’ acid generationpotential and were conducted in view of the possiblepresence of acid-generating sulfide minerals within the

contaminated soils. Soil subsamples were sievedthrough 500-, 250- and 100-μm stainless steel sieves,and the mass fractions of the sieved portions weredetermined gravimetrically.

Soil and fertiliser samples were ground in a chromesteel ring mill in preparation for chemical and X-raydiffraction (XRD) analysis. Mineral identification ofnon-amended soils and fertilisers was performed atthe University of Newcastle EM/X-ray Unit usingsemi-quantitative powder XRD.

Soil and fertiliser powders were digested in a hotHF–HNO3–HClO4–HCl acid mixture and analysed byinductively coupled plasma mass spectrometry(ICPMS) and inductively coupled plasma atomicemission spectrometry (ICPAES) for their near-totalAs, Cd, Cu, Fe, Mn, P, Pb, Stotal, Sb and Zn contents

Fig. 1 Map of Broken Hilland Mount Isa withsampling sites for soilsBH1, BH2 and MS1

Water Air Soil Pollut (2012) 223:2237–2255 2239

at Australian Laboratory Services (ALS; Brisbane).Total carbon (Ctotal) and organic carbon (Corg) con-centrations in soil and fertiliser samples were deter-mined using a Leco furnace, while carbonate carbon(Ccarb) was calculated by difference between the Lecomethods (ALS, Brisbane). Sulfide sulfur (Ssulfide) wasanalysed as chromium-reducible S (Southern CrossUniversity, Lismore). Sulfate sulfur (Ssulfate) wascalculated by difference between the total sulfur(Stotal) and sulfidic sulfur (Ssulfide) concentrations.

2.3 Laboratory Experiments and Analyses

Phosphate stabilisation tests were conducted using free-draining leach columns (polymethyl methacrylate; 1 min height; 6 cm in diameter). Columns were filled withsoil to approximately 900 mm in depth (≈ 2.6–3.0 kg)on a bed of nylon gauze, filter paper (Whatman 541) and15-mm pure quartz sand. SuPer pellets (<2 mm) wasadded to three soil columns (labelled MS1+P, BH1+P,BH2+P) to constitute 0.5 wt.% of each soil mass (13–15 g SuPer). The amount of superphosphate addedcorresponds to approximately 50 t/ha. Fertilisers wereadded as a mixed soil and fertiliser layer in the top 5 cmof columns to simulate field application where only veryshallow mixing is practicable. Three control columnsreceived no SuPer (labelled MS1, BH1, BH2). The deepsoil columns provided an experimental system aimed atinvestigating chemical reactions with depth and do notrepresent actual soil profiles at the sampling sites.Replication of column treatments was not undertakenas the high number (ten) of monthly analyses of columnleachates provided an assessment of whether data setswere internally consistent and allowed a high level ofconfidence in any observed differences betweenphosphate-amended and non-amended columns. Fur-thermore, analysis of five replicate samples of each ofthe prepared soils demonstrated that their major metalcontent varied by less than 8%.

The leaching experiments used repeated wetting–drying cycles to stimulate oxidation and flushing ofoxidation products (Smart et al. 2002). In the initialsetup, air was removed from the soil columns bysaturation with deionised water (DIW) to reduce theformation of preferential flow canals within the col-umns. During this procedure, soil columns werecompacted by approximately 10–15% due to grainpacking. Following initial setup, the columns weresubjected to a drying period of approximately 30 days

under heating lamps (8 h/day). Wetting–drying cycleswere repeated nine times over a total period of 310 dayswith 500 mL of DIW added each month. This volumewas selected to produce adequate leachate for analyses.The proportion of DIW that drained through thecolumns varied from 44% to 65% for the six columnsover the 10-month test duration. Following collection ofleachate, pH was recorded and leachates were filtered(0.45 μm Gelman Acrodisc), acidified to 1% HNO3

(Merck SupraPur) and analysed for As, Cd, Cu, Fe,Mn, P, Pb, S, Sb and Zn by ICPMS and ICPAES atthe James Cook University Advanced AnalyticalCentre (JCU AAC, Townsville, Qld, Australia).

Upon completion of the leaching experiment, thesoil columns were opened and sectioned with respectto column depth (0–5 cm, 5–10 cm and then every10 cm for the remaining column). Samples weredried, pulverised and analysed for their near-total As,Cd, Cu, Fe, Mn, P, Pb, Stotal, Sb and Zn contents asdetailed above for the bulk soil samples.

Soils from the 5–10-cm-deep sections of the non-amended columns and from the 0–5- and 5–10-cmsections of the fertiliser-amended columns weremounted in polished resin blocks and element mapsof a 10 × 10-mm area obtained using a JEOLJXA8200 electron probe micro analyser (beam size=20 μm; step size 20 μm) at JCU AAC. Selected mapswere analysed for their grain size distribution usingthe software programme ImageJ (NIH 2010).

2.4 Quality Control of Geochemical Data

Elemental analyses carried out at ALS (NATAaccredited as ISO/IEC 17025 compliant) and JCUAAC included replicate analyses, blanks, spike additionrecovery and certified reference materials (CRM).Replicate determinations agreed to within ±10% and,spike and CRM recoveries were in the 80–120% range.The analytical precision of chemical data is estimated tobe within ± 10% at concentration levels more than fivetimes the detection limit.

3 Results

3.1 Soils

Soil BH1 is silty clay loam with 53 wt.% <0.1 mmand contains high Mn, Pb and Zn and elevated As, Cd

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and Sb concentrations compared to average soils(Smith and Huyck 1999). Sulfidic sulfur constitutes75% of Stotal (Table 1), indicating that this soil has asubstantial amount of sulfides and is only partiallyoxidised. Quartz, feldspar, mica and Mn–garnetdominate the coarser-sized fractions and quartz, micaand kaolinite dominate the fine-grained fractions ofsoil BH1 (Munksgaard and Lottermoser 2011). SoilBH2 is a silty loam with 22 wt.% <0.1 mm. Metalsand metalloid concentrations in BH2 are lower than inBH1 (Table 1), but sample BH2 is still characterisedby high Mn, Pb and Zn and elevated Cd and Sbvalues compared to average soils (Smith and Huyck1999). Sample BH2 is almost fully oxidised asdemonstrated by its low Ssulfide/Stotal ratio of 3.3%(Table 1). The mineralogy of BH2 is similar to that ofBH1 but with the addition of gypsum as a minorphase. Minor kintoreite (PbFe3(PO4)2(OH, H2O)6)and coronadite (PbMn8O16) were identified by XRDin both BH1 and BH2. Soil pH and NAG pH testsdemonstrate that BH1 and BH2 are not net acid-producing (Table 1). Also, leachates derived from thesoils and collected during the laboratory experimentsexhibited near-neutral pH values over time (Fig. 2),

indicating that the investigated soils have no potentialto produce acidity within several months.

Soil MS1 is silty loam with 38 wt.% <0.1 mm and ischaracterised by high Cu, Pb and Zn; As and Cd are alsosignificantly elevated. MS1 is partly oxidised as shownby a Ssulfide/Stotal ratio of 76%; Ctotal and Corg

concentrations are substantially higher than in BH1and BH2 (Table 1). XRD analysis of soilMS1 identifiedthe presence of quartz, feldspar, mica, clinochlore andkaolinite. Soil pH and NAG pH tests demonstrate thatMS1 is weakly acid-generating (Table 1). Also, leach-ates collected during the laboratory experimentsdemonstrate that the soil produced acidic leachatesafter approximately 3 months of leaching (Fig. 2).This acid production occurs despite its acid bufferingcapacity (Ccarb=6.5 g/kg).

No Fe-, Pb- or Zn-bearing sulfides, sulfates orcarbonates were identified by XRD in the three soilsinvestigated. However, electron microprobe phasemapping indicated the presence of fine-grainedmaterial containing Zn and S as well as Pb and S(sulfide or sulfate phases). High detection limits inpowder XRD (up to several vol% depending onsample matrix) and fine grain size commonly impede

Table 1 Chemical composition of soils (sulfide-rich BH1 and MS1; sulfate-rich BH2) and superphosphate amendment (SuPer) usedfor the laboratory experiments. Australian HIL for residential (A) and industrial (F) soils are also shown (NEPC 1999)

Unit BH1 BH2 MS1 HIL A, F SuPer

pH (solid/water) 7.3 7.8 5.9 − 3.33

pH (NAG) 6.9 8.0 6.3 − na

Ctotal g/kg 6.2 1.9 38.0 − 3.0

Corg g/kg 4.3 1.0 31.5 − 2.1

Ccarb g/kg 1.9 0.9 7.5 − 0.9

Stotal g/kg 2.0 9.2 2.1 − 120

Ssulfide g/kg 1.5 0.3 1.6 − 3.4

Ssulfate g/kg 0.5 8.9 0.5 − 117

P g/kg 1.4 0.8 0.7 − 94.3

Fe g/kg 57.1 50.6 42.4 − 3.6

As mg/kg 90.6 42.4 64.7 100, 500 14

Cd mg/kg 22.5 10.3 19.4 20, 100 23.9

Cu mg/kg 155 97.5 2,920 1,000, 5,000 20.7

Pb mg/kg 12,500 5,520 2,350 300, 1,500 14.6

Mn mg/kg 63,900 44,300 551 1,500, 7,500 139

Sb mg/kg 93.7 48.2 9.44 − 2.27

Zn mg/kg 7,060 4,670 1,070 7,000, 35,000 240

Data of BH1, BH2 and SuPer from Munksgaard and Lottermoser (2011). pH (solid/water) was 1:5 for soils and 1:100 for fertiliser

− not analysed

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mineral identification in phosphate stabilisation stud-ies (Chrysochoou et al. 2007). Hence, the XRD datado not preclude the presence of significant contents ofmetal sulfides, sulfates, phosphates or other phases inthese soils. For example, the elevated Ssulfide contentof the Mt Isa soil and its acid soil and leachate pHvalues demonstrate that this soil contains acid-generating pyrite or pyrrhotite which are common inMount Isa ores (Forrestal 1990; Perkins 1990).

Concentrations of Mn in BH1 and BH2 and of Pbin MS1, BH1 and BH2 exceed the Australian HealthInvestigation Levels (HIL) for both residential andindustrial soils (HIL A and F, respectively) (NEPC1999). Copper concentrations in MS1 exceed HIL Alevels and Cd concentrations in MS1, and both Cdand Zn concentrations in BH1 are similar to HIL Alevels (Table 1).

3.2 Fertiliser

SuPer fertiliser is acid-generating (pH 3.33) asdetermined by 1:100 fertiliser/water extraction(Munksgaard and Lottermoser 2011). Dissolution ofSuPer in water released detectable quantities of metalsand metalloids (Munksgaard and Lottermoser 2011).However, the absolute quantities of trace metalscontributed from the fertiliser in the amended soiltests are insignificant relative to the soil contributionsbecause the soils contain much higher concentrationsof most metals compared to SuPer and addition ofSuPer amounted to only 0.5% of the total soil columnmass.

3.3 Chemical Composition of Soil Leachates

The leachates of the non-amended BH1 and BH2soils showed near-neutral pH values (approximate-ly 6.6–8.2; Fig. 2) during progressive leachingcycles. SuPer amendment caused no substantialchanges in leachate pH compared to the non-amended soils (Fig. 2). By contrast, leachates ofnon-amended MS1 became acidic from the fifthleachate (pH 5.2) before a return to neutral pH inthe final leachate portions. The first five leachateportions of SuPer-amended MS1 showed similar pHvariations to the non-amended soil. However, theremaining leachate portions all had low pH values(4.4 to 4.9).

The elemental concentrations of leachates arecommonly used as a criterion to assess the perfor-mance of phosphate treatments (e.g. Harris andLottermoser 2006a, b; Chrysochoou et al. 2007).Detailed views of the leaching behaviour of elements(As, Cd, Cu, Fe, Mn, Pb, S, Sb, Zn) in MS1, BH1 andBH2 are shown in Fig. 3a–i. These figures show thecumulative mass of leached elements (derived fromleachate concentrations and volumes) as a function ofthe cumulative leachate volume. This presentation ofthe data has the advantage of directly comparingelement release from amended and non-amendedleach tests without the influence of variable leachvolumes on element concentrations (Munksgaard andLottermoser 2011). The leachate analyses are inter-nally consistent and hence indicate element trends.The metals of main environmental interest (As, Cd,

4.0

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Cu, Pb, Sb and Zn) showed the following features(refer to Fig. 3 for other elements):

As: Leachates of the non-amended MS1 soilshowed a high As release in the second leachateportion followed by a reduced release. In SuPer-amended MS1, a similar pattern occurred but As

release was higher and occurred in the firstand second leachate portions. Non-amendedBH1 and BH2 soils released low levels of Asthroughout the experiment. Arsenic releasewas substantially increased in the SuPer-amended BH1 soil but not in SuPer-amendedBH2.

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Fig. 3 Leachate chemistry of non-amended (MS1, BH1, BH2) and amended soils (MS1+P, BH1+P, BH2+P). a As. b Cd. c Cu. d Fe.e Mn. f Pb. g S. h Sb. i Zn

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Cd: Non-amended MS1 showed an initial releaseof Cd, but from the fourth leachate portion therelease increased and persisted to the finalportion. In SuPer-amended MS1, the samepattern occurred but the released mass increasedthree- to fourfold. Leachates of non-amended andamended BH1 showed a similarly high and near-constant release of Cd throughout the test. Bycontrast, leachates of non-amended and amendedBH2 initially contained high Cd with very littlefurther release in subsequent leachate portions.Cu: MS1 soil contains relatively high Cu con-tents (Table 1), and there was an elevated releaseof Cu in the first leachate portion of non-amended and amended soils followed by lowreleases. From the sixth portion in both tests,there were high releases of Cu, and this washigher in the amended soil compared to the non-amended soil by a factor of approximately 3.Leachates of non-amended and amended BH1

and BH2 soils, both Cu-poor relative to MS1,showed an initial small release of Cu followed byalmost no further discharge in the remainingleachate portions. For both soils, the initial Cumobilisation was higher in the SuPer-amendedtests compared to the non-amended tests.Pb: Non-amended MS1 showed little Pb mobilisa-tion throughout leaching. However, SuPer-amendedMS1 showed a near-constant release of Pb in theseventh to tenth leachate portions. Non-amendedBH1 leachates showed a high Pb release in the fifthto tenth portions. SuPer-amended BH1 also showedPb release in the last four leachate portions butsubstantially less Pb was mobilised in this testcompared to the non-amended BH1 soil. Bothleachates of non-amended and amended BH2showed a high Pb release in the initial leachateportion with very little further discharge after that.The initial Pb release was higher in the SuPer-amended test compared to the non-amended test.

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Fig. 3 (continued)

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Sb: Non-amended MS1 showed Sb release in thefirst two leachate portions followed by a lowerand constant release. Leachates of SuPer-amendedMS1 showed a similar pattern of Sb release, but themass of Sb released was approximately double thatof the non-amended MS1 test. Non-amended BH1leachates showed a near-constant Sb discharge overthe duration of the experiment. In the leachates ofSuPer-amended BH1, Sb was substantially higherthan in the non-amended BH1 test. Leachates ofnon-amended and amended BH2 showed similarsmall releases of Sb throughout the experiment.Zn: Leachates form non-amended MS1 initiallycontained little Zn, but from the fifth leachateportion the release increased and persisted to thefinal portion. The same release pattern occurredin SuPer-amended MS1, but the level of Znrelease was two- to threefold higher than in thenon-amended soil. Leachates of the non-amendedand amended BH1 soil had significant Zn fromthe fourth leachate portion. Compared to MS1and BH1, BH2 leachates showed much lowerreleases of Zn in all leachate portions.

Phosphorus released from all non-amended andSuPer-amended soils were near or below detectionlimits (3 mg/L) in all leachate portions. Chrysochoouet al. (2007) recognised the need for excess of P insolution to effectively scavenge Pb, which desorbs anddissolves from Pb-bearing minerals. Non-amendedMS1,BH1 and BH2 soils had total molar P/(Cu+Zn+Cd+Pb)ratios of 0.32, 0.27 and 0.35, respectively. In the SuPer-amended soils, these ratios were raised to 3.8–3.9.However, low P concentrations in leachates suggest that

reactive phosphate is unlikely to have been available inthe deeper soil sections.

Sulfur release in all six leaching tests was nearlyconstant throughout each experiment (Fig. 3g). In thenon-amended soils, BH2 released more S thanMS1 andBH1, as can be expected based on the high sulfatecontent of BH2 (8.9 g/kg; Table 1). There wereincreased S discharges in all three SuPer-amendedsoils. The cumulative molar S/(Cu+Zn+Cd+Pb) ratiosin leachates of non-amended MS1, BH1 and BH2were 45, 5.3 and 866, respectively. These ratios are inexcess of molar S/metal ratios found in commonmetal-bearing sulfide and sulfate minerals and indi-cate substantial dissolution of non-metal bearingsulfides and/or sulfates.

The concentrations of metals, metalloids, P and Sin the final leachate portions collected at completionof the fertiliser treatment tests (Table 2) provide an‘end-point’ indicator of the effectiveness of thefertiliser treatments. In field applications, similar datawould be used as a measure of treatment performanceagainst water quality criteria (cf. Chrysochoou et al.2007). Compared to leachates of the non-amendedsoils, the following observations can be made on thefinal leachate portions of the SuPer-amended soils:

1. There were no substantial changes in the Pconcentrations.

2. The S concentrations were elevated in BH1 butremained approximately unchanged in MS1 andBH2 leachates.

3. Changes inmetal andmetalloid concentrations weremainly seen in MS1 leachates, where amendmentcaused increases in Cu (×10), As (×10), Cd (×4) and

Table 2 Chemical composition of final leachate portion (no.10) at the conclusion of the 10-month leaching test of non-amended and phosphate-amended (+P) soils. Trigger values

(TV) for slightly to moderately disturbed freshwater ecosystemsare also shown (NWQMS 2000)

Soil test Fe, mg/L Mn, mg/L Cu, μg/L Pb, μg/L Zn, mg/L Cd, μg/L As, μg/L Sb, μg/L S, mg/L P, mg/L

MS1 1.9 45.6 753 24 24 269 8 3.4 394 <3.0

MS1+P 2.1 37.3 7,590 1,050 26 666 77 5.1 375 3.0

BH1 0.7 0.2 24 2,360 145 1,070 3 3.6 167 6.5

BH1+P 1.9 1.1 34 2,720 170 1,410 3 2.6 413 5.3

BH2 1.9 0.005 15 9 0.1 11 2 1.0 531 4.1

BH2+P 1.9 0.02 12 7 0.1 14 2 1.2 544 <3.0

NWQMS TV nv 1.9 1.4 3.4 0.008 0.2 94, 42 nv nv 0.01

The first As value shown is for AsIII ; the second is for AsV

nv no trigger value defined

Water Air Soil Pollut (2012) 223:2237–2255 2245

Pb (×44) concentrations. In BH1, SuPer amendmentcaused increases inMn (×5), Fe (×3) and S (×2). Bycontrast, no substantial changes in metal andmetalloid concentrations were detected in the finalleachates of BH2.

The rate and timing of metal and metalloid releasevaried substantially over the duration of the differenttests. When calculated as cumulative mass release overthe entire duration of the leaching trials (Table 3), theSuPer-amendedMS1 soil displayed an increased releaseof Cu (×3), Zn (×2), As (×2), Cd (×3), Sb (×2), Pb (×9)and P (×4) compared to the non-amended soil MS1.Amendment of BH1 resulted in increased releases ofMn (×2), Cu (×3), As (×8), Sb (×2), S (×2) and P (×2).In BH2, SuPer amendment caused a greater mobilisa-tion of Cu (×2). By contrast, there were substantialreductions in cumulative mass releases of Fe in MS1(×4) and Pb in BH1 (×2) due to SuPer treatment.

3.4 Chemical Composition of Leached Soil Columns

Comparisons of the initial mass of elements present inthe soils with the cumulative mass of elementsreleased during leaching demonstrate that less than1% of the initial mass of most metals and metalloids(As, Cu, Fe, Mn, Pb, Sb) was mobilised from the soilcolumns. Only in a few cases was the proportion ofelements leached distinctly higher (2.4% of Zn and4.2% of Cd from MS1; 1.4% of Zn and 5.0% of Cd inBH1; 7.9% of Cd in BH2).

Sulfur was enriched in the top 40 cm of the SuPer-amended MS1 column compared to the non-amendedcolumn, whereas the SuPer-amended BH1 columnwas enriched in S only in the 0–5-cm sectioncompared to the non-amended column (Fig. 4a, b).The S enrichments were due to the SuPer amendment

containing anhydrite (CaSO4). In the BH2 column, Sconcentrations were substantially lower in the 0–20-cm section of the non-amended BH2 column com-pared to those at greater depths (Fig. 4c). This isconsistent with the presence of soluble gypsum inBH2 and the release of high levels of S in BH2leachates. In the SuPer-amended BH2 column, therewas a markedly higher S concentration in the 0–10-cm interval relative to the same interval in the non-amended BH2 column due to the addition of fertiliser(Fig. 4c). As expected, large enrichments of P inSuPer-amended soils relative to non-amended soilswere observed in the upper parts of the soil columns(MS1, 65 cm; BH1, 25 cm; BH2, 25 cm; Fig. 5a–c).Differences in bulk metal and metalloid concentra-tions in column sections between non-amended andSuPer-amended tests were in most cases smaller thanthe analytical uncertainty (± 10%), and definite trendsin metal and metalloid composition with depth couldnot be identified.

3.5 Microprobe Element Mapping of Amended Soils

Microprobe element mapping of amended soilsidentified a number of changes to soil compositionsresulting from SuPer amendments. These composi-tional changes were evidenced by observed differ-ences in element correlations in non-amended andamended samples. Element correlations for Zn and Pbversus P and S were plotted for BH1 and BH2 withadditional Cu versus P and S plots created for MS1.Each plot consisted of 250,000 pixel values (countsper second) for each 10×10-mm probed area. Quasi-linear element correlations can result from variableproportions of grains of a distinct mineral phase beingencountered by the electron beam across the grainmount. Although the phases may remain unidentified,

Table 3 Cumulative mass release at the conclusion of the 10-month leaching test of non-amended and phosphate-amended (+P) soils.Phosphorus values below the detection limit were calculated as half the detection limit

Soil test Fe, mg Mn, mg Cu, mg Pb, mg Zn, mg Cd, mg As, mg Sb, mg S, mg P, mg

MS1 27.2 77.3 4.6 0.21 35.4 0.74 0.44 0.023 902 7

MS1+P 7.5 61.0 15.2 1.91 67.3 2.12 0.83 0.040 1,040 29

BH1 6.3 5.7 0.18 5.09 280 3.07 0.01 0.011 736 6

BH1+P 8.5 10.9 0.46 3.32 293 3.33 0.10 0.025 1,110 11

BH2 10.1 0.11 0.78 3.51 1.71 2.06 0.04 0.005 2,040 8

BH2+P 8.7 0.09 1.20 4.42 1.21 2.19 0.04 0.006 2,330 6

2246 Water Air Soil Pollut (2012) 223:2237–2255

0.0

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Fig. 4 Sulfur concentrationsof non-amended andfertiliser amended soils(a MS1, b BH1, c BH2)upon completion of theleaching experiments

Water Air Soil Pollut (2012) 223:2237–2255 2247

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Fig. 5 Phosphorus concen-trations of non-amendedand fertiliser amended soils(a MS1, b BH1, c BH2)upon completion of theleaching experiments

2248 Water Air Soil Pollut (2012) 223:2237–2255

changes in element correlations are likely to reflectchanges in grain size and/or mineralogy associated withfertiliser-induced chemical reactions (Munksgaard andLottermoser 2011). The main changes observed insoils BH1, BH2 and MS1 were:

1. SuPer amendment of BH1 and BH2 showed similarincreases in Pb–P (Fig. 6a, b) and Zn–P (Fig. 6c, d)correlations compared to the non-amended soils,especially at low count rates, suggesting thedissolution of pre-existing Pb and Zn grains andthe precipitation of new fine-grained Pb- and Zn-bearing phosphate phases.

2. Linear Pb–S and Zn–S correlations wererelatively strong in the non-amended sulfide-rich BH1 soil (due to the presence of primarygalena and sphalerite) but were absent in thesulfate-rich BH2 soil. SuPer amendment ofBH2 led to a stronger linear correlation of Zn–S (Fig. 6e, f), indicating the formation of a newZn–S phase.

3. Non-amended MS1 soil showed a strong linearCu–S correlation (due to the presence of primarychalcopyrite), which was also present in the amendedMS1 soil sections (0–5 and 5–10 cm). SuPeramendment produced a second strong linear Cu–Scorrelation with a relatively low S/Cu slope in the 0–5-cm section (but not in the 5–10-cm section)(Fig. 6g, h), demonstrating the development of anew Cu–S phase.

4. A small increase in the number of pixels with bothCu and P was observed in amended MS1 comparedto non-amended MS1. Yet there were no substantialcorrelations of Pb and Zn with either P or S in non-amended or amended MS1 soils.

The grain size distribution in selected elemental mapswas analysed using ImageJ software (NIH 2010).Several algorithms are included in the software to selectthe threshold signal above which a pixel is retained inthe threshold-limited map and the grain size analysis;the choice of method can strongly influence the resultsof the analysis. Several threshold methods were readilyexcluded due to visual mismatch of grain shapes in thethreshold-limited map and the original map. Twothreshold methods (‘Default’ and ‘Otsu’; NIH 2010)provided good matches between threshold-limited andoriginal images. The grain size analysis based on thesethreshold methods resulted in consistent relative changes

in grain size distribution between non-amendedand SuPer-amended soils. The analysis showed: (a)that >75% of grains in all three soils with an above-threshold signal of Pb and P were in the 1–5 pixel sizerange (corresponding to 20–100-μm-sized grains) and (b)that the main changes in the number of Pb- and P-richgrains predominantly occurred in the smallest (1–5 pixels)grains when soils were amended with SuPer (Fig. 7a–d).

4 Discussion

4.1 Phase Reactions in Amended andNon-amended Soils

Phosphate stabilisation of metal-contaminated soilsand wastes can be viewed as a three-stage process: (1)dissolution of soluble, metal-bearing phases that needto be stabilised, (2) dissolution of a phosphate sourceand (3) precipitation of insoluble metal phosphates. Inthe case of Pb stabilisation, the principal mineral oflow solubility is one of the forms of pyromorphite[Pb5(PO4)3X] (X=OH, Cl, Br, F). Cao et al. (2002,2003, 2008) and Chrysochoou et al. (2007) consid-ered that optimal conditions for the formation ofinsoluble pyromorphite initially require acid pHconditions (pH∼5) to instigate Pb dissolution fol-lowed by rising pH values to facilitate precipitation ofpyromorphite. Note that the pH requirement for initialmetal dissolution does not allow the use of alkalinefertiliser amendments. By contrast, acid-producingfertilisers, such as superphosphate, may assist initialdissolution of the existing metal phases in neutralsoils. However, acid production must decline or bepartly neutralised for precipitation of insoluble metalphosphates to become significant.

In the column tests reported here, fertiliser additiondid not result in a substantial pH decrease in soilsBH1 and BH2, which is due to the ability of thesesoils to buffer acidity. However, amendment of soilMS1 with SuPer resulted in pH reduction of approx-imately 0.5–2 units compared to the non-amended soiland the reduced pH was maintained throughout theremaining test period. Two of the soils investigated werenot net acid-producing (BH1: pH 6.5–7.5; BH2:pH 7.5–8) and MS1 was only weakly acid-producing(minimum pH 5.2). Consequently, the necessary pHconditions for optimal phosphate stabilisation (Cao et al.2002, 2003, 2008; Chrysochoou et al. 2007) were notachieved in BH1 and BH2 as there was no significant

Water Air Soil Pollut (2012) 223:2237–2255 2249

initial pH reduction to assist metal dissolution.Furthermore, in SuPer-amended soil MS1, pH didnot rise in the final stages of the experiment to assistpyromorphite precipitation.

The mobilisation of some metals as metal–organiccomplexes may have been an additional factor prevent-ing the formation of metal orthophosphate in theamended columns. Complexation of metals by soilfulvic and humic acids is influenced mainly by thecompetition for binding between metal ions and protonsand electrostatic effects (Tipping 1998). Such a disso-lution behaviour can play an important role in promot-ing leaching of metals from soils (Martinez et al. 2002;2004), especially for Cu and Pb, which form strongorganic complexes even at a relatively low pH (Lawlorand Tipping 2003). Munksgaard and Lottermoser(2010) established that amendment of mining-impactedsoils with pine bark accelerated the release of metals andmetalloids due to a combination of pH decrease andmetal complexation by soluble organic ligands. In thisstudy, the abundance of organic matter in MS1 (Corg=31.5 g/kg) is likely to have caused a discharge oforganic acids as evidenced by lowering of pH valuesto 5.2 and 4.4 in leachates of non-amended andphosphate-amended soil, respectively. By contrast, theorganic C content of soils BH1 (Corg=4.3 g/kg) andBH2 (Corg=1.0 g/kg) is much lower than in MS1, andBH1 and BH2 leachate pH did not decrease below 6.5in spite of their very low acid buffering capacity. Thisindicates that production of organic acids and theconcentration of soluble organic ligands, capable ofaccelerating metal and metalloid release, were low insoils BH1 and BH2.

Although metal-bearing phases could not beidentified using XRD, microprobe element mappingdemonstrated the formation of newly precipitated Pb–P and Zn–P phases in the top sections (0–10 cm) ofthe fertiliser-amended BH1 soil. Image grain analysesshowed that the mineralogical changes were predomi-nantly induced in the smallest grain size fraction.Moreover, there was reduced Pb release into soilleachates (Fig. 3f). Therefore, mineral reactions,known to stabilise metals via phosphate precipitation(e.g. Zhang and Ryan 1999a,b; Ryan et al. 2001),were initiated in the SuPer-amended BH1. Micro-probe element mapping also demonstrated that newZn–S phases precipitated in the SuPer-amended BH2soil and that new Cu–S phases appeared in theamended MS1. These secondary phases were

likely relatively soluble secondary sulfates, whichformed during the drying/evaporation cycle of theleaching tests. Such sulfates would be susceptibleto dissolution and therefore do not lead to metalstabilisation.

4.2 Element Mobility in Amended and Non-amendedSoils

To date, acid mine drainage testing and metalmobilisation studies commonly employ small-sizedleach columns over relatively short time periods (e.g.Smart et al. 2002). By contrast, this study relied onprolonged leaching over 10 months of deep soilcolumns resulting in repeated drying/wetting cycles(10). These experimental conditions allowed therecognition of contrasting trends in the mobility ofseveral metals and metalloids in fertiliser-amendedand non-amended soil treatments (Fig. 3).

The initial release of relatively high levels of As, Cd,Pb and S from the non-amended soil BH2 demonstratedthe presence of soluble metal-bearing sulfate efflores-cences. However, the substantial reduction in metalmobilisation in the remaining part of the leaching testshowed that most of the remaining metal content of thesoil is hosted by relatively insoluble phases. Phosphateamendment of BH2 did not substantially alter therelease of metals, possibly due to the rapid initial releaseofmetal andmetalloids from sulfate efflorescences. Thisdischarge preceded the availability of soluble phosphatewhich had to be released and transported from theSuPer-amended surface layer. Although previous stud-ies found that a relatively slow releasing phosphatesource is necessary to ensure effective ongoing stabili-sation of metals (Cao et al. 2002, 2003), the data forBH2 highlight the need for a highly soluble phosphatesource in soils containing abundant soluble mineralefflorescences.

In the sulfide-rich soils BH1 and MS1, the releaseof most metals and metalloids was substantially lowerbut, once initiated, continued at near-constant rates for

Fig. 6 Elemental relationships (counts per second) as deter-mined by electron microprobe mapping in Mt Isa and BrokenHill soils. Samples were collected from non-amended (5–10 cmin depth) and amended soil columns (0–5 cm in depth). a Pb–Pin BH1. b Pb–P in BH1+P. c Zn–P in BH1. d Zn–P in BH1+P.e Zn–S in BH2. f Zn–S in BH2+P. g Cu–S in MS1. h Cu–S inMS1+P. Linear trends indicate the presence of discrete mineralphases

b

2250 Water Air Soil Pollut (2012) 223:2237–2255

Water Air Soil Pollut (2012) 223:2237–2255 2251

the duration of the tests (Cd, Sb, Zn in BH1; Cd, Cu,Pb, Sn, Zn in MS1). The amendment of BH1 andMS1 with SuPer fertiliser increased the total releasedmass of several metals and metalloids (Mn, Cu, Asand Sb in BH1; Cu, Zn, As, Cd, Sb, Pb in MS1). Insoil MS1 with 76% of Stotal being sulfidic, metalrelease appears to have been assisted by proton

dissolution following the pH drop (pH minimum=5.2; Fig. 2) during the latter half of the leaching tests.In soil MS1, sulfide oxidation led to a minor net acidproduction (cf. Evangelou and Zhang 1995). In theSuPer-amended MS1 test, mobilisation of metals (Cd,Cu, Pb, Zn) and metalloids (As, Sb) was furtherenhanced by an additional pH decrease due to

0

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Fig. 7 Size distribution ofgrains with above-thresholdPb and P count rates innon-amended andSuPer-amended BH1 andBH2 soils. The thresholdcount rates were determinedby the ImageJ ‘Default’threshold algorithm(NIH 2010). Pixel size is20×20 μm

Fig. 6 (continued)

2252 Water Air Soil Pollut (2012) 223:2237–2255

fertiliser-derived acidity (pH minimum=4.3; Fig. 2).By contrast, in both the amended and non-amendedBH1 soil, pronounced metal (Cu, Mn) and metalloid(As, Sb) mobilisation occurred at near-neutral pH(6.5–7.5). Such trace element mobility is likely due tothe high solubility of Cu sulfate, the lack of abundantCcarb (Table 1) that would favour the precipitation ofMn and the dissolution of metalloids (As, Sb) asoxyanions in porewaters at near-neutral pH conditions.

Prolonged leaching of the soil columns allowedsoluble phosphate to be delivered to deeper soil levelsas evidenced by an increase in soil P concentrations atdepth at the conclusion of the leaching tests (Fig. 5).A simultaneous availability of soluble metals andphosphate is necessary to enable the formation ofinsoluble metal phosphates, for example, the forma-tion of pyromorphite (e.g. Zhang et al. 1997, 1998;Ryan et al. 2001). The reduction in cumulative Pbrelease from the phosphate-amended BH1 soil com-pared to non-amended soil is a likely example of thismechanism. However, a similar reduction in Pb dis-charge was not observed in the amended MS1 and BH2soils. In fact, the cumulative mass of Pb released intoleachates increased moderately in BH2 and increasedsubstantially in MS1 after SuPer amendment. Suchincreases in Pb releases are possibly due to organiccomplexing of metals as well as pronounced sulfideoxidation and associated soil acidification and enhancedmobilisation of metals.

4.3 Limitations of Phosphate Amendment

The concentrations of several metals (Cd, Cu, Mn, Pband Zn) in the three mining-impacted soils from BrokenHill and Mount Isa were above residential and, in somecases, industrial health investigation levels (Table 1).Furthermore, prolonged leach testing showed thatconcentrations of metals and metalloids in subsurfacedrainage waters from some of these soils are likely toexceed Australian ecosystem guideline levels forsurface waters (NWQMS 2000). A comparison ofthe final leachate concentrations to current Australianguideline values for metals and metalloids in fresh-water ecosystems (NWQMS 2000) shows that Mn,Cu, Zn, Cd and Pb concentrations in the leachate wereelevated to a substantial degree above the guidelines(by more than three orders of magnitude in somecases). Although the final leachates of soil BH2exceeded NWQMS guideline concentration levels

for some metals (Cu, Zn, Cd, Pb), relatively lowdilution rates (by a factor of approximately 10) inreceiving waters would be sufficient to negate toxicityimpacts. By comparison, final leachates of soils BH1and MS1 exceeded NWQMS guideline concentrationlevels for several metals (Mn, Cu, Zn, Cd, Pb) by twoto three orders of magnitude and would requiresubstantial dilution to negate toxicity impacts inreceiving waters. Therefore, leachates of amendedsoils could constitute a toxicity threat to any receivinggroundwater or open freshwater bodies unless theyare substantially diluted before off-site release.

Arsenic and Sb concentrations in BH1, BH2 andMS1 did not exceed the Australian Health Investiga-tion levels for soils (NEPC 1999). However, As andSb release was substantially increased in the fertiliser-amended BH1 and MS1 soils. Soil As (mainlyarsenate) and Sb are known to be desorbed by ligandexchange of H2PO4

− or HPO42− and to form soluble

oxyanions in surface environments (Alam et al. 2001;Lombi et al. 2000; Wilson et al. 2010). Therefore,phosphate treatment is likely to cause As and Sbrelease from mine wastes (Harris and Lottermoser2006a,b), firing range soils (Spuller et al. 2007;Kilgour et al. 2008) and mining-impacted soils asdocumented by this study.

5 Conclusions

Column experiments conducted over a 10-monthperiod explored the mobility and potential stabilisa-tion of metals and metalloids in mine waste-impactedsoils using superphosphate fertiliser. The tests dem-onstrate that fertiliser amendment had highly variableeffects on the degree of metal and metalloid mobi-lisation and capture. In soils with abundant organicmatter, metal/metalloid complexing with solubleorganic acids led to pronounced metal and metalloidrelease from fertiliser-amended soil. In pyrite-richsoils, acidity and metal mobility (mainly Cd, Cu, Zn)increased over time. In addition, metalloids (As, Sb)were increasingly released from phosphate-amendedsoils under acid to neutral pH conditions. By contrast,fertiliser application to a sulfide-rich soil buffered atnear-neutral pH and with low organic carbon contentresulted in the immobilisation of Pb in the form ofnewly precipitated Pb phosphate phases. These find-ings show that both the oxidation state of sulfur (i.e.

Water Air Soil Pollut (2012) 223:2237–2255 2253

sulfide versus sulfate) and the amount of organicmatter have a direct influence on the solubility ofmetals in mine waste-impacted soils. These factors arealso likely to control metal bioavailability andphytotoxicity as shown by Martinez et al. (2002).

Consequently, phosphate stabilisation was an inef-fective method for the treatment of polymetallic waste-impacted soils that were rich in organic matter andcontained abundant pyrite, which in turn generatedpersistent unbuffered acidity. Phosphate stabilisationappears to be more effective for the in situ treatment ofsoils that are distinctly enriched in Pb and containinsignificant concentrations of organic matter and othermetals and metalloids.

Acknowledgements This research was supported underAustralian Research Council’s Discovery Projects fundingscheme (project number DP0877182). Dr. Yi Hu (JCU AAC) isthanked for analytical support.

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