Arsenic removal from water by iron-sulphide minerals

NOTESArsenic removal from water by iron- sulphide mineralsHAN Jingtai1,2 & William S. Fyfe21. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Department of Earth Sciences, the University of Westerm Ontario, London, Ontario N6A 5B7, CanadaCorrespondence should be addressed to Han Jingtai (-mail: jhan@ julian. uwo.ca)Abstract In bench-scaled experiments, iron-sulphide minerals, pyrite and pyrrhotite are used asadsorbents for arsenic removal from As-spiked water of As+ and As+ species. The adsorption rate,efficiency，As-adsorption stability and the associated pH conditions have been examined.Observations indicate that these iron-sulphide minerals are very efficient to adsorb arsenic fromwater for both As5+ and As3+ species. Similar to other studies, As3+-adsorption shows a slower ratethan As3+. The stability of the adsorbed arsenic seems closely related to the pH values of thesolution. A lower pH level commonly less than 4.0 is required to protect the adsorbed arsenic fromserious oxidation and backward release. Fining of the mineral powders and shaking of the solutionduring adsorption enhance the adsorption efficiency and adsorption rate. For practical use of themethod presented in this study, the waste produced should be managed with great care to keep itfrom redistribution over water system. A further study of the protection for the waste from oxidationon real water systems will greatly enhance the application of the strong ability of arsenic adsorptionby these minerals, which is observed from this study.Keywords: arsenic contamination, arsenic removal, ironsulphide minerals.Arsenic contamination of drinking water has been reported from many parts of the worldincluding Asia, Europe and America along the belt of low latitudes. It causes a variety of diseasesparticularly skin cancer12. Detailed studies of arsenic contamination of water system and theassociated arsenism in the areas of Xinjiang, China have been carried out for years [3]. Arsenic poisoningin Bengal and western India occurred in recent years, where millions of people are poisoned or in therisk of arsenic poisoning, is claimed as“the worst hydrogeological problem in the world", or“theworld' s biggest environmental health disaster"H. The recent literature is flooded with such reportsl4“8The World Health Organization suggests to lower the limits of 0.05 mg/L arsenic of the old standard tobe less than 0.01 mg/L9, and the US Environmental Protection Agency proposes a further lower limitof 0.002 mg/L5]. The World Bank has recently announced a US$32.4 million, zero-interest loan toBangladesh for work to reduce the problemt 10]. With increase in coal consumption and mining of goldand other metal ores in China, which are the major sources of arsenic pollution, arsenic contaminationof the environment will certainly be a big potential problem, and thus studies of arsenic pollution andtreatment of removal are in great importance for human health and economic development of the entirenation.Numerous studies of arsenic removal from drinking water and waste waters use methodsinvolving chemical precipitation such as alum and iron precipitationl “13), lime softeningl4.15],membranesl16l, colloidal flotationl7), adsorption by activated iron and aluminall3], and ion exchangeresin[16]. However, all the available treatments have some defects either in their efficiency or wastedisposal problems they producedllo, or expensive equipments required, which poses the difficulties fortheir application for areas under poor economic conditions. This study try to search for some geologicmaterials practical in use to remove arsenic from water, which has the potential in large scales oftreatment of arsenic removal from water systems.Arsenic is frequently associated with sulphide minerals and typically forms its own minerals likearsenopyrite (FeAsS)I8), so that arsenic commonly cnpantrntad in mon; tnes of sulphide mineraldeposits with gold. Some extreme examples of pol中国煤化工arelessagemennt ofmine waste as in the Mediterranean[19l. On theC N M H G arsenic with sulphideminerals raises a potential use of sulphide minerals I0r arsenic removal rrom water. In this study, weshow the possibility of these minerals in practice to adsorb arsenic from arsenic-spiked waters throughbench-scaled experiments.1430Chinese Science BulletinVol.45 No. 15August 2000NOTES1 Materials and methodsAs+-spiked water was prepared from sodium hydroarsenate (Na2HAsO4 7H2O) with deionizedwater. Pyrite (FeS2) and pyrhotite were put into the solution as arsenic adsorbents. Before quantitativeexperiments, the crystals of pyrite and pyrrhotite were used to test their ability in arsenic adsorption.After a few hours of exposure with the water containing arsenic and then rinsed with deionized waterand dried up, the minerals were examined using Auger and X-ray photoelectron spectroscopy. It wasfound that the surface of the minerals was coated with a layer of arsenic. For quantitative analysis,pyrite and pyrrhotite powder with size-controls was used in the subsequent experiments.Pyrite and pyrrhotite powder less than 300 mesh, and pyrite powder plus iron filings made threeconsecutive measurement lines; each line consists of 25 samples. Each individual sample contains 100mL of 10 mg/L As-spiked water with addition of the powder adsorbent, 0.4g pyrite, or 0.4g pyrrhotite,or 0.3g pyrite plus 0.2g iron. During the first 5 days, all samples were shaken for 5 min for every 8 h.Afterwards, shaking was made once a day. One sample of each line was measured each day. Formeasurement of arsenic concentration, the supernatants were separated using a centrifuge (20 000 r/minfor 20 min) and analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) with .a low detection limit of 0.005 mg/L.To examine the adsorption ability of these minerals to As3+, the As 3+-spiked water was made usingsodium arsenite (NaAsO2) and deionized water. The initial arsenic concentration was 10 mg/L. Theadsorbents，methods and procedures were the same as used in the above experiments. The pHmeasurement was made using hydrogen-electrode pH meter with a precision less than 0.2. Allchemicals were purchased from VWR Inc. The standard solutions of arsenic and sulfer were providedby the ICP-OES Instrument Company, and checked with the standards made by the authors.2 Results and discussionThe consecutive measurements ofAssconcentration for the watersamples treated with pyrite, pyrrhotite198085and pyrite plus iron filings are plottedin fig. 1 and listed in table 1. Obviously,pyrite adsorbs arsenic faster thanpyrrhotite; after 5 days all the arsenic is01removed from the water (at least thearsenic concentration is lower than the0r=? F.......... r086oe80lower detection limit of the instrument).。PyriteAt the 8th day, however, the adsorbed10~●Pyrie+t'e .arsenic begins serious release to the口Pyrrhotite .water. About 20 days later, the arsenic_in the solution rises to a level close to51002:the initial concentration. From the line1 ime/dof pyrite with addition of iron filings， Fig. 1. Changes of arsenic concentaions in spiked waters with time aferthe backward release of the adsorbed adding iron- sulphide minerals. The arsenic concentrations of 0.005 mg/Larsenic is significantly reduced. With are in fact equal to or lower than the ICP -OES detection limit.the lapse of time more and more rust isproduced. In another experiment, however, we found that iron filings virtually have no ability to adsorbarsenic. These could suggest that the backward release of adsorbed arsenic by pyrite be subjected tooxidation. Iron filings provide some help for maintaining 9 rerdwrine cnnditin for the solution.Compared with pyrite, pyrrhotite adsorbs ar中国煤化工1 and table 1) but theadsorption is much stable. Only a very slight backwIYHC N M H Garsenic is detected (roseto 0.01 mg/L) after 6 months. It has been observed'ual ue arsenc cwincenuation of the spiked waterdecreased proportionally with the increase in sulfer (data not shown). This suggests that arsenicadsorption results from replacement of sulfer on the mineral surface by arsenic and formation ofarsenopyrite. During the arsenic adsorption by pyrrhotite, the formation of each arsenopyrite moleculeChines雨$e敝掘e Bulletin Vol. 45 No. 15August 20001431NOTESalso produces a free iron, which changes the mineral surface structure and allows arsenic to penetrateinto the depth of the mineral. By this mechanism, the adsorption process could continue until allavailable arsenic is consumed provided that the amount of the mineral suffices. We also observed thatthe pyrrhotite powder in all the samples forms a solid layer coating on the bottom of the beakers after aperiod of time. In contrast, pyrite powder always remains loose, indicating that the arsenic adsorption ismainly restricted on the surface. The free iron produced during arsenic adsorption by pyrrhotiteeffectively maintains a reducing condition for the water and thus protects the adsorbed arsenic fromoxidation and backward release. Because pyrite is enriched in sulfer than pyrrhotite, it could providemore sulfer for substitution by arsenic during the surface adsorption process, given the same amountand the same grain size of these minerals. Therefore, pyrite shows a higher adsorption rate thanpyrrhotite.Table 1 Arsenic concentrations (mg/L) in spiked waters after As+ removal by three powder adsorbentsTime/dPyritePyrrhotitePyrite+ FeyritePyrite + Fe010126.81t. 0.0050.0050.3036.740.29717.54:.. 0.005.. 0.0050.0185.780.0227.72”0.005"- 0.00530.0135.360.0151:8.01.0.00540.0104.860.0060.0074.390.0141'0.0210.52:. 0.005188.15:.0.0050.0080.460.094198.31..0.0050.0124.58i.0.00528.37218.44.5.058.52. ..0.005|15.92The As concentrations less than :. 0.005 mg/L are in fact lower than the ICP-OES detection limit.Comparative studies of AsS+ and As3+ adsorption by both minerals (the data are listed in table 2)are ilustrated in fig. 2. As concluded from many studies'"I “13.16, the adsorption rates of As3+ aresignificantly slower than those of As+ at the first stage. Nevertheless, both arsenic species can becompletely removed by pyrite and pyrrhotite after about 10 days. Therefore, As3+ may not benecessarily oxidized into AsS+ first and then to be cleaned up if a natural water system is treated and theadsorption efficiency is pursued most.Table 2 Comparison of adsorption rate of As3+ with AS'+ by pyrite+Fe and pyrthotite powdersPyrite+FeTime/dayAs (..) .As( .)As..ly .As(_)5.490.0524.982.780.4390.1920.221“-*0.0050.082“"0.005--0.0050.009The As concentrations less than F 0.005 mg/L are in fact lower than the ICP-OES detection limit.During the experiments for construction of fig. 1, we also observed dramatic changes in the pHvalue of the As-spiked water samples. In the early stage when the adsorbed arsenic remains stable, i.e.the arsenic concentration of the solution falls down中国煤化工/el, the pH values of allthe samples are less than 4.0. During the period ofYHCNMHdsorbed arsenic, the pHvalue rises to a level higher than 7.0. This change (_ward release. Therefore,the pH value is an important factor for arsenic removal by both pyrite and pyrrhotite and for thestability of the adsorbed arsenic.1432Chinese Science BulletinVol.45 No. 15August 2000NOTESShaking can significantly affecthe adsorption rate. For samplesprepared as described in fig. 1, 24 h of田Pyrite-FelIAs'!shaking is sufficient for pyrite to adsorb●Pyrite . Fe-[As*t}all the arsenic or for the arsenic蠹Prtrhotitc-[As' ]concentration to fall down to a level口Prrhotite-[As°I]lower than the low detection limit.Fining of the adsorbent powder0"-enhances their adsorption efficiency, butalso increases difficulty for separationof the solids from water. Both factors， 10-2 Fshould be considered in order tooptimize the treatment using this10~method.Compared with other geologicmaterials like hematite， feldspar, and0912Time/dclay as adsorbents to remove arsenicfrom water20, we found that pyrite and Fig. 2. Comparison of adsorption rate of As3+ with As+ by pyrite+Fe andpyrrhotite have much higher efficiency，pyrrhotite. The arsenic concentrations of 0.005 mg/L are in fact equal to orat least two orders greater in magnitude. lower than the ICP- OES detection limit.Arsenic speciation study[2n) indicates that arsenic in the natural fresh water systems is dominated byAs+. However, arsenic contamination occurring in Bengal and western India contains a considerablyhigher portion of As+1221. For arsenic removal from a real water system, the arsenic species involved inthe system should be specified and be well understood. There are some available methods that can beused to convert As+ to As+113], for example, the chloride oxidation method, if such a treatment isnecessary. When using adsorption method presented in this study to remove arsenic from drinkingwater, the solid waste produced must be managed with great care, because oxidation of the sulphideminerals containing arsenic is a main path for arsenic mobilization and redistribution in naturalsystems [231.3 ConclusionsIron sulphide minerals like pyrite and pyrrhotite are effective adsorbents for arsenic removal fromwater, thus have great potential in future practical treatment of arsenic polluted water. Compared withother adsorption materials， for example， lime， ferric chloridel24, and other minerals examinedpreviously, iron sulphide minerals produce less waste and the waste is more easily separated from water.In practice, however, the waste disposal should be safe enough to protect the adsorbed arsenic frommobilization and redistribution over the environment. The strong ability of iron sulphide minerals inarsenic adsorption provides a new way for arsenic removal. Once a proper management for thesewastes is achieved on a large scale, practical use of these minerals for arsenic removal will be enhancedgreatly.Acknowledgements This work was supported by an NSERC grant to WSF. Z. Y. Gu and anonymous reviewers are gratefullyacknowledged for reviewing the manuscript.References1. Pearce. F, Arsenic in tapwater linked to skin cancer, New Scientist, 1993, 30: 5.2. Gorby, M. S., Arsenic in human medicine, Arsenic in the Environment, Part . : Human Health and Ecosystem Effects (ed.Nriagu, J. O.), New York: John Wiley & Sons, Inc., 1994, 1 "8.3. Wang, L, Huang, J, Chronic arsenism from drinkingChina, in Arsenic in theEnvironment, Part - : Human Health and Ecosystem Effe中国煤化I1994, 9 "16.4. Kaiser, J,, Toxicologists shed new light on old poisons, Sci:YHCNMH G,27 *9..5. Lepkowski, W., Arsenic crisis in Bangladesh, Chemical &6. Gebel, T. W, Arsenic and drinking water contamination, Science, 1999, 283: 1458.7. Nickson, R., McArthur, J.. 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