Removal of Lead, Copper, Zinc and Cadmium from Water Using Phosphate Rock

Vol. 82 No. 6 pp.1223. -1228ACTA GEOLOGICA SINICADec. 2008Removal of Lead, Copper, Zinc and Cadmium from WaterUsing Phosphate RockAlessia CORAMI', Silvano MIGNARDI.2.* and Vincenzo FERRINI.21 Department of Earth Sciences, University "La Sapienza", P.le A. Moro 5, 00185 Rome, ltaly2 IGG-CNR (Sezione di Roma), co Department of Earth Sciences, University“La Sapienza", P.le A. Moro 5, 00185 Rome, lialyAbstract: Removal of Pb"*, Cu"*, Zn2+ and Cd2*+ from aqueous solutions by sorption on a naturalphosphate rock (FAP) was investigated. The effects of the contact time and initial metal concentrationwere examined in the batch method, The percentage sorption of heavy metals from solution rangesgenerally between 50% and 99%. The amount of sorbed metal ions follows the order Cu>Pb>Cd>Zn.Heavy metal immobilization was attributed to both surface complexation of metal ions on the surfaceof FAP grains and partial dissolution and precipitation of a heavy metal-containing phosphate. Thevery low desorption ratio of heavy metals further supports the effectiveness of FAP as an alternativeand low-cost material to remove toxic Pbt, Cu2*, Zn2* and Cd2+ from polluted waters.Key words: heavy metal ions, phosphate rock, sorption1 Introductionand cost (e.g., Bossrez et al, 1997; Yu and Kaewsarn,1999). Adsorption is a method that can be economicallyRapid industrialization and urbanization has resulted inused for heavy metals removal from contaminated waterhe deterioration of water, air and land quality. Th(e.g.. Ozer and Piringgi, 2006). The main disadvantages ofincrease in the use of heavy metals over the past fewadsorption processes are high costs of the most adsorbentsdecades determined an incrcasing flux of metallicrecommended. Therefore, cheaper and readily availablesubstances in the environment. The environmentalmaterials to be used as adsorbents have been investigated.pollution by toxic metals is worth special concern becauseSome of the low-cost adsorbents used for removal ofof their carcinogenic properties, bio-accumulation andheavy metals are, for example, hazeInut shell, perlite, crabnon-biodegradability (Perez-Marin et al, 2006). Heavyshell, bone char, eggshell, waste lea, low- grade phosphate,metals such as lead (Pb), copper (Cu), zinc (Zn) andPinus bark, montmorillonite (e.g., Cimino et al, 2000; Ancadmium (Cd) belong to the group of serious hazardouset al., 2001; Cheung et al, 2001; Mathialagan andheavy metals (e.g.. Adriano et al., 2004; Qin et al, 2006)Viraraghavan, 2002; Vazquez et al, 2002; Kandah, 2004;and may adversely affect water quality, soil ecology andChojnacka, 2005; Malkoc and Nuhoglu, 2005; Liu et al,agricultural production (e.g.. Drake and Rayson, 1996;2006).Han et al, 2000; Gravilescu, 2004). The origin of thisSynthetic or natural apatite and phosphate rock havepollution derives mainly from various industries such asbeen proved to effectively immobilize heavy metals frommetal finishing, electroplating, mining, storage batteries,contaminated waters and soils (e.g.. Suzuki et al, 1981;glass, ceramic, paper and paint manufacturing (e.g., Han etMa et al, 1993, 1994; Aklil et al, 2004; Cao et al, 2004;al，2006). Heavy metals pollution represents anSaxena and D'Souza, 2006; Qi and Liu, 2006; Marchat etunacceptable public health hazard and must be remediated.al, 2007). The objective of these studies is to establishDuring recent years several methods for wastewaterless soluble heavy metal-containing mineral phases thattreatment have been developed such as chemicalare more geochemically stable in a wide range ofoxidation, precipitation, reduction,solidification,enviro. vious work (Coramimembrane systems, electrolytic recovery, ion exchangeet al,中国煤化工iveness of syntheticand adsorption (e.g., Ozverdi and Erdem, 2006; Nasir ethydro:MHCNMHGymetalsinsolutionsal., 2007). However, such methods vary in effectivenesshas been investigated. The results indicate that* Corresponding author. E-mail: silvano.mignardi@ uniromal.ithydroxyapatite is able to substantially remove heavy1224Removal of Lead, Copper, Zinc and Cadmium from Water Using Phosphate RockCorami et al.Table 1 Chemical composition of FAPWeight (2)WeighL (x10)P2O527.81Cd3SiO211.61Cu742.12Ni12200 um0.99In23Pb1440CoPb>Cdno new different shaped precipitates have been identified>Zn for all initial metal ion concentrations. This order is inon the surface of FAP grains. The solid residues showagreement with those reported in previous studies (e.g,white zones spread on the FAP grains (Fig. 1C),Aklil et al, 2004; Cao et al, 2004).corresponding to the sorbed heavy metals (McCrellis etHeavy metals retention by FAP produces a very lttleal, 2001) and confirmed by EDS analyses which detectedincrease of Ca concentration in the solution, ranging fromthe presence of the heavy metals on the surface of FAPvalues below ICP-AES detection limit to 0.83 mg/L.grains.Generally, Ca amount in the solution increases withcontact time, whereas with the increasing of the initial3.3 Removal of Pb2+, Cu+, Zn2+ and Cd2+ by FAPmetal concentration the amount of Ca released decreases.The results of the sorption experiments are given inTable 2. The percentage of removed Pb2+ ranges from3.4 Q, molar ratio67.80% to 99.85% (9.82- -67.80 mg/g). For Cu2+ theThe evaluation of the heavy metal immobilization hasamount of immobilized metal ranges from 89.65% tbeen inferred on the basis of the chemical analysis of the99.55% (9.96 -77.89 mg/g). The percentage of blocked Cdsolutions before and after sorption experiments. Aranges from 51.74% to 98.73% (9.87- 51.74 mg/g). .parameter which characterizes the sorption mechanism isFinally, as concerns Zn the amount of removed metalthe molar ratio Qr It is defined as the molar ratio betweenranges normally from 23.28% to 97.87% (4.66 -9.79 mg/the cations bound by FAP to Ca desorbed by FAP. If Q.=g). However, it is interesting to notice that for Zn at theI, the quantity of sorbed and released cations is the same,highest initial metal concentration (500 mg/L), FAP is .indicating the possibility of ion exchange of cationsunable to immobilize the metal. Table 2 shows that whenbetween the FAP and the solution. When Qs > 1, thethe initial metal concentration is increased from 10 to 500amount of Ca released is less than the amount of sorbedmg/L, the removal capacity of FAP increases and themetal suggesting surface complexation as the mainpercentage sorption generally decreases (excluding Zn), insorption mechanism. When Q. < I, dissolution of FAP andagreement with the results of previous studies (e.g.,precipitation of new phosphate phases with lower cation toAhmad et al, 2003; Saxena and D'Souza, 2006). Thephosphate molar ratio occurs.increase of removal capacity with increase in initial metalOur values (Table 2) were < 1 at low initial metalconcentration is a result of the concentration gradient inconcentration and become 》1 as the concentrationthe solution. The decrease in percentage sorption suggestsincreases. These data suggest two different sorptiona decrease in the number of binding sites at the FAPmechanisms: surface complexation and the dissolution ofsurface available for the sorption as much more metal ionsFAP and precipitation of a new phosphate phase. Similarare present in the solution. At low concentration, almostconclusions have also been inferred in previous studiesall heavy metal ions in solution could interact with the(e.g.. Da Rocha et al, 2002; Aklil et al, 2004; Corami etactive sites, whereas the saturation of the binding sites atal, 2007).high concentration results in a lower sorption yield. In thisview, in wastewater treatment by phosphate rock an3.5 Sorption mechanismincrease in the amount of removed metals can be obtainedMetal immobilization by phosphate minerals may occurby diluting the solutions containing high metal ionby ion exchange, surface complexation, dissolution oconcentrations or using a larger amount of amendment tophosphate minerals and precipitation of new metalincrease the number of available binding sites.phosphates, and subtitution of Ca in phosphate mineralsThe effectiveness of FAP in immobilizing heavy metalsration (coprecipitation)was found apparently depending on the initial metal ion(e.g.中国煤化工al, 14; Xuet al,concentration. The amount of sorbed mass follows the1994C N M H Gver, the dominance oforders: Cu>Cd>Pb>Zn (Co= 10 mg/L), Pb>Cu>Cd>Znone of ne aoove-menuonea mecnanisms is rather difficult(Co = 100 mg/L) and Cu>Pb>Cd (Co = 500 mg/L),to distinguish as it seems that they could often act togetherVol. 82 No.6ACTA GEOLOGICA SINICADec.20081227Table 3 Desorbed metal concentration and desorption ratio of heavy metals sorbed onto FAPHNO,Desorbhed melal concentration (mg/L)__ Desorplion ratio (密) Desurbed metal concentration (m/L)Desorption ratio (%)ptd1 0.78 0.100.92PhCu2nCdPbCPbCu∠nCd<0.1 0.19 0.24 0.46bdl 0.48 0.40 0.43<0.1 0.12 0.17 0.18dI bdl 0.10 0.72<0.1 0.16 <0.1 0.18bd0.96b2.29<0.1 0.24 <0.1 0.52dl bll_0.10 0.21<0.1 <0.1 <0.1 <0.1bu0.660.85<0.1 <0.1 0.23 0.27bl = bclow delection limit(e.g.. Cao et al, 2004; Peld et al, 2004).4 ConclusionsThe patterns in Fig. 2 suggest that the sorption processinvolves two steps: a rapid surface complexation onFAP is effective in immobilizing Pb2+, Cu2+, Zn2+ andbinding sites of FAP, followed by the formation of heavyCd2+ in aqueous solutions. Percentage sorption rangesmetal-containing phosphates, in agreement with the resultsgenerally from 50% to 99% and decreases with increasingof previous studies (e.g.. Da Rocha et al, 2002; Aklil etthe initial metal concentration in the conditions of thisal, 2004; Cao et al, 2004).study. In this view, in the treatment of polluted watersXRD, SEM-EDS and AFM analyses did not detect anyusing phosphate rock an increase in the sorption yieldnew erystallinephosphates, suggestingthat thecould be achieved by diluting the wastewater containingdissolution/precipitation was not the main sorptionhigh heavy metal concentrations or using a larger amountmechanism. The observed slight increase of Caof amendment to increase the number of sites available forconcentrations in the solutions further supports that thethe sorption. The amount of sorbed metals follows thecontribution of dissolution/precipitation mechanism to theorder Cu>Pb>Cd>Zn. The proposed mechanism involvesoverall sorption was very small.both surface complexation of heavy metals on the surfaceFrom our Q3 values < I is inferred that at low initialof FAP grains and the partial dissolution and precipitationmetal concentration the contribution of the dissolution/of a heavy metal-containing phosphate. The very lowprecipitation mechanism is meaningful. With increasing ofdesorption ratio of heavy metals further supports thethe initial metal concentration, our Q,》I suggest thateffectiveness of FAP in removing heavy metals fromthe surface complexation is the main sorption mechanism.polluted waters.The increase of Q，values can be atributed to theformation of a heavy metal-containing phosphate layerAcknowledgementsonto the grains which limited further FAP dissolution(Valsami-Jones et al, 1998). Therefore, the proposedThis study corresponds partly to the Ph.D. research ofoverall sorption mechanism involves both dissolution/A. Corami. Financial support was partially from the CNR-precipitation and surface complexation.IGG-Roman Branch. The authors thank T. Coppola and S.Stellino for their laboratory assistance.3.6 Desorption of sorbed metalsDesorption of sorbed heavy metals from FAP dependsManuscript received July 9, 2007on pH of the extracting solutions (Table 3). 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