This article examines the removal of heavy metals (Pb, Cr, Cu, Zn) from aqueous solutions using ordinary sand as an adsorbent. Batch experiments were conducted by adding sand to metal salt solutions and measuring metal concentration after 24 hours. The data fit the Langmuir adsorption isotherm model well. The maximum amount of metal adsorbed to form a monolayer (am) was highest for Pb and lowest for Zn, indicating Pb's stronger interaction with sand. This preference is attributed to the metals' relative abilities to hydrolyze, with more easily hydrolyzed ions (like Pb2+) favoring chemisorption to silicate surface sites. The study concludes that while sand

1. ~" -
,
Jolunalof Eooironmental
SciencesVol. 15.No.3.pp.413-416.2003 ~:11~~;:~2
Article m: 1001-0742(2003)03-0413-04 CLC number: X131;X506 Document
cooe: A
Removal 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. awan@mail.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. was
studied at 20~. The amountof metal adsorbedto form monolayeron sand( am)' obtainedfrom Langmuir isotherm. exhibited the preferenceof
metals for sand in the order Ph > Cr > Cu > Zn. The heavy metal-sandadsorptionphenomena
can be illustrated on the basis of the interaction
betweensurfacefunctional group of silicates (sand) and the metal ions. It is deduced that sand can be used as a low cost adsorbentfor tbe
removal of heavy metal from wastewater(containing low conc. of metals). especially in the developingcountries.
Keywords: sand; heavy metals; adsorption
Introduction
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 dischargedinto
natural waters. The harmful effects of such metals on living being are well known. Chromium exists in as Cr3+ and Cr"+ in
water. Theseare biologically critical species(Sawyer. 1994; Dara. 1997); Cr"+ is carcinogenicin nature whereaslong-term
exposure 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 been
classified as water pollutants. Consequently. removal of heavy metals from wastewater and industrial wastes has become a very I
important environmental issue. The process of adsorption is considered one of the most suitable methods of the removal of
contaminants from water and a number of low cost adsorbents have been reported for the removal of heavy metals (ions) from
aqueoussolutions. 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 adsorption
efficienciesfor different metals.
In addition to the above adsorbents. soils and clays also show remarkahlepotential towardsthe removal of heavy metals
ions 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 + and
Zn2+ 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. +- am
Where: 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) adsorbateadsorbedto
form 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

2. 414 M. Ali Awan et 01. Vol. IS
1 Material and method
All heavy metal salts i. e.. chromium chloride, copper sulfate, lead acetateand zinc sulfate (Merk) were of analytical
grade 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 each
metal 225 rnl of the solution was taken in 250 ml Erlenmeyerflask and 10 g sand was added (concentrationof sandwas thus
44 .45 g/L). The pH of each solution was adjusted to around 6.0 by adding a few drops of NaOH solution. At this slightly
acidic pH a metal precipitation doesnot occur. The flasks in triplicate were cappedand shakenon the flask shakerat 50 r/min
for 24 hours at 20~. The solution, abovethe sandwas filtered and its metal concentration, C, (mg/L) was determinedusing
the 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 isotberms
me s as s own III Ig. to respectivey . ereas Ig. 1. e. x m vs £ Imtl
conc. 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 1.68
Cr> Cu> In. Among the four metalsPb is most readily absorbed sandwhile Zn is
on Cu2+ 0.144 2.45
the 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 be
helpful. Silica (SiO2) has a structure composed infinite three-dimensional
of frameworkof tetrahedron(Murray, 1994). Each
silicon atom forms four single bondswith four oxygenatomslocated at the four comers of a tetrahedron. Like silica, feldspars
are 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:

3. 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
/ the'initial 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 metal's 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.
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(Received for review March 18, 2002. Accepted May 9, 2002)