~" - ,Jolunalof Eooironmental SciencesVol. 15.No.3.pp.413-416.2003 ~:11~~;:~2Article m: 1001-0742(2003)03-0413-04 CLC number: X131;X506 Document cooe: ARemoval of heavy metals through adsorption using sand ;M. Ali Awan I, Ishtiaq A. Qazi2, Imran Khalidl(1 .Institute of Environmental Science and Engineering (lESE). National University of Sciences & Technology (NUsr). Rawalpindi.Pakistan. E-mail: ali. email@example.com; 2. Pakistan Council of RenewableEnergy Technologies. Islamabad. Pakistan)Abstract: The removal of four heavy metals i. e. Pb. Cr. Cu. and Zn from their aqueoussolutions. using ordinary sand as an adsorbent. wasstudied at 20~. The amountof metal adsorbedto form monolayeron sand( am) obtainedfrom Langmuir isotherm. exhibited the preferenceofmetals for sand in the order Ph > Cr > Cu > Zn. The heavy metal-sandadsorptionphenomena can be illustrated on the basis of the interactionbetweensurfacefunctional group of silicates (sand) and the metal ions. It is deduced that sand can be used as a low cost adsorbentfor tberemoval of heavy metal from wastewater(containing low conc. of metals). especially in the developingcountries.Keywords: sand; heavy metals; adsorptionIntroduction Water contaminants include a large number of chemicals ranging from aromatic hydrocarbons. organic solvents.pesticidesand metals. Heavy metal contaminationis commonlyfound in areas where industrial effiuents are dischargedintonatural waters. The harmful effects of such metals on living being are well known. Chromium exists in as Cr3+ and Cr"+ inwater. Theseare biologically critical species(Sawyer. 1994; Dara. 1997); Cr"+ is carcinogenicin nature whereaslong-termexposure to Cr3+ can cause allergic skin reactions leading to cancer. Similarly. copper is extremely toxic to aquatic biota (U .S. EPA. 1985). Lead (Manahan. 1991) and zinc(Diamond. 1993; Murray. 1994; Evangelou. 1998) have also beenclassified as water pollutants. Consequently. removal of heavy metals from wastewater and industrial wastes has become a very Iimportant environmental issue. The process of adsorption is considered one of the most suitable methods of the removal ofcontaminants from water and a number of low cost adsorbents have been reported for the removal of heavy metals (ions) fromaqueoussolutions. I Activated charcoal is very efficient in removalof metal ions. but is readily soluble under very low and high pH conditions(Huang, 1989). Peat moss was used for the removal of mercury. copper, cadmium (Coupal. 1976) and nickel (Ho.1995). Claymperes delessertti(moss) was used for the adsorptionof copper from aqueoussolution (Lee. 1989). Also lead.cadmium and zinc were adsorbedon wastetea leaves (Tee. 1988). The adsorbentsdiscussedaboveshow different adsorptionefficienciesfor different metals. In addition to the above adsorbents. soils and clays also show remarkahlepotential towardsthe removal of heavy metalsions from water (Murray, 1994; Evangelou, 1998). Removal of heavy metals by slow sand filters has also been reported(Schmidt. 1997; Muhammad, 1997; 1998). In the present work adsorption potential of ordinary sand towards selected heavy metal ions i. e. Cr3 +. CU2 + , Pb2 + andZn2+ has been examinedby plotting the Langmuir adsorptionisothermsand extracting the necessaryparametersfrom theme. Langmuir adsorptionisotherm: The Langmuir adsorptionisotherm. applicable to solutions(adsorbate)can be expressed as(Jaffer, 1983): mil I x - amK C. +- amWhere: C, is the concentrated(mg/L) of adsorbate(metal) at equilibrium; X is the concentration (mg/L) of adsorbate(metal) adsorbedat equilibrium; m is the concentration(g/L) of adsorbent;a.. is the amount (mg/ g) adsorbateadsorbedtoform a monolayer; and K is the Langmuir constant. A plot of ~ against-C gives a straight line from the intercept and slope l x.of which am and K can be calculated. Here the units of the ratio ~ is g/mg. x * Corresponding author
414 M. Ali Awan et 01. Vol. IS1 Material and method All heavy metal salts i. e.. chromium chloride, copper sulfate, lead acetateand zinc sulfate (Merk) were of analyticalgrade and were used without further purification. 500 g, ordinary sand was collected from a riverbed near Rawalpindi,Pakistan. It is washedseveraltimes with distilled water and left to dry in open air. Separatesolutions, ranging in concentrationfrom 1-5 mg/L, were preparedfor each metal in distilled water. For eachmetal 225 rnl of the solution was taken in 250 ml Erlenmeyerflask and 10 g sand was added (concentrationof sandwas thus44 .45 g/L). The pH of each solution was adjusted to around 6.0 by adding a few drops of NaOH solution. At this slightlyacidic pH a metal precipitation doesnot occur. The flasks in triplicate were cappedand shakenon the flask shakerat 50 r/minfor 24 hours at 20~. The solution, abovethe sandwas filtered and its metal concentration, C, (mg/L) was determinedusingthe Atomic Absorption Spectrophotometer, Varian, model: AA-375.2 Results and discussion Langmuir isotherm !!!:.- vs -CI gave a linear relation (straight lines) for all four Table 1 Adsorption data obtained x , tal h . F. I 4 . I Wh F. 5 . / C.( . . .al from Langmuir isotbermsme s as s own III Ig. to respectivey . ereas Ig. 1. e. x m vs £ Imtlconc. of metal) for Cr3 exhibitsan increasing + trend. Tahle I illustrates data the Metal am, K,calculated from the isothermsfor all the four metals. mg/g Umg Pb2+ 0.169 0.11 The values of am exhibit the preferenceof metal ions for sandin the order Pb> C;+ 0.148 1.68Cr> Cu> In. Among the four metalsPb is most readily absorbed sandwhile Zn is on Cu2+ 0.144 2.45the least readily adsorbed. Zn2+ 0.131 2.51 In order to understand the above metal-trend of adsorption on sand,considerationof the types of associations among adsorbingmetal ions and silica and feldspar (componentsof sand) may behelpful. Silica (SiO2) has a structure composed infinite three-dimensional of frameworkof tetrahedron(Murray, 1994). Eachsilicon atom forms four single bondswith four oxygenatomslocated at the four comers of a tetrahedron. Like silica, feldsparsare also frameworkof silicates. 40 25 -t! 20 -t! , " 30 ;" ..- ---"-" ~: ~ ~ 10 10 5 0 0 0 I 2 3 4 0 I 2 3 4 5 tICs, L/mg tICs, L/mg Fig. I Langmuir adsorption isotherm Pb2 ontosandat 20t: for + Fig.2 Langmuir adsorption isotherm C; + ontosand 20t: for at 25 16 ,..-..- 20 12 -t! IS ~ ---~ -t! ~ ~ 8 10 . 4 5 00 I 2 3 4 00 5 tICs, L/mg tICs, Umg Fig.3 Langmuir adsorptionisothermfor Cu2+ onto sand at 20t: Fig.4 Langmuir adsorptionisotherm for Zn2+ onto sand at 2Ot:
No 3. Removalof heavy metals through adsorptionusing sand 415 A surface functional group in silicates plays a 0.6 significant role in the adsorptionprocess. It is a plane of O.s oxygen atoms bound to the silica tetrahedral layer and 0.4 .. hydroxyl groups that are associatedwith the edges of the E 0.3 silicate structural units (Donald. 1998). Thesefunctional 0.2 .d groups proVI e Sunace sites lor thh c emlsorphon 0f " r e " 0.1 0 transition and heavy metals (Murray. 1994). The surface 0 2 4 6 8 10 12 functional groups can be representedas: Ct, mg/L " S-O H . Fig.5 x1m (amount C~+ adsorbedper gram of sand) increaseswith of / theinitial conc. C, of C~+ Where S is central atom (Si or AI) of adsorbingsurfaceof silicates (Donald. 1998). The surface hydroxyl groups dissociatein water and serve as Lewis bases towards metal cations (MO + ). Such deportonated sites (one or possibly two) fonns complex with the heavy metal ions as follows (Murray. "" / S-OH + Mo+~ / S-OM(o-I)+ + H+, 1994). ~I " .,; 2 "S-OH + Mo+ ~ "(S-0)2M(o-2)+ + 2H+ . / / Heavy metal cations Mo + may also hydrolyze in aqueous solution and adsorb in a hydrolyzed fonn according to the following reaction. Mo+ +H2O~M(o-I)+OH+H+(Hydrolysis). " S-OH +M(o-)+OH~ " S-OM(o-2)+ OH+H+(Adsorption). / / The metal-surface bonding(adsorption)reactionis favoredby the metals properties favor its hydrolysis.Such that properties includehighcharge,smallradiusandpolarizability (Murray. 1994). Though the values of am for all the four metals do not differ much yet the reason of slightly higher value for Pb2+ is that silica adsorbs it a bit strongly as it (Pb2+ ) can easily be hydrolyzed in water (Murray. 1994). which in turn favors its chemisorptions on silicates. C? + , due to its trivalent nature adsorbs bit more than the two other divalent metals (CU2 + and Zn2 ). It is. therefore. can be deduced that adsorptionand hydrolysis are somewhatcorrelated; if the heavy metal ion is + easily hydrolysable, its adsorption on silicates is relatively higher as in the case of Pb2+ . The sand after adsorption becomes saturated with the adsorbed metals. It can be disposed off by employing it as a component of concrete used in various construction purposes. In a study Vallejo et ai. proposed a method of immobilization of industrial residues containing zinc and chromium using two types of cements (Vallejo. 1999). 3 Conclusion The linear trend of the plots ~ vs -l for Pb. Cr, Cu and Zn confinn that their adsorptionon sand also follows the C x . Langmuir adsorptionmodel. The removal capacity of sand for heavy metals in aqueous solutions is related to their (metals) potential towards hydrolysis. Thoughordinary sand adsorbsonly a small amount of heavy metals yet it can be used as a low cost adsorbentfor their removalfrom wastewaters (containing low conc. of such metals). especially in the developingcountries. Acknowledgements: Assistanceof Mr. MuhammadBasharatand Idrees Ahmad of lESE in metal analysisis acknowledged. References: Coupal B. LalancetteJ M. 1976. The treatmentof wastewaters with peat moss[J]. Wat Res. 10: 1071-1076. Oars S S, 1991. Environmentalchemistry and (KIllution control[ M]. New Delhi: S. Chand & CompanyLtd.c~C"",".
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