Environ Earth Sci (2013) 68:641–645 DOI 10.1007/s12665-012-1767-z

ORIGINAL ARTICLE

Adsorption kinetics of chromium(III) removal from aqueous solutions using natural red earth Chandima Nikagolla • Rohana Chandrajith Rohan Weerasooriya • C. B. Dissanayake

•

Received: 15 February 2011 / Accepted: 5 June 2012 / Published online: 22 June 2012 Ó Springer-Verlag 2012

Abstract Laboratory-scale-simulated experiments were carried out using Cr(III) solutions to identify the Cr(III) retention behavior of natural red earth (NRE), a natural soil available in the northwestern coastal belt of Sri Lanka. The effects of solution pH, initial Cr(III) concentration and the contact time were examined. The NRE showed almost 100 % Cr(III) adsorption within the first 90 min. [initial [Cr(III)] = 0.0092–0.192 mM; initial pH 4.0–9.0]. At pH 2 (298 K), when particle size ranged from 125 to 180 lm the Cr(III) adsorption data were modeled according to Langmuir convention assuming site homogeneity. The pH-dependent Cr(III) adsorption data were quantified by diffused layer model assuming following reaction stoichiometries: þ 2 [ AlOHðsÞ þ Cr ðOHÞþ 2 ðaqÞ ! ð[AlOÞ2 CrðsÞ

þ 2H2 O

log K 15:56

þ 2 [ FeOHðsÞ þ CrðOHÞþ 2 ðaqÞ ! ð[FeOÞ2 CrðsÞ

þ 2H2 O

log K 5:08:

The present data showed that NRE can effectively be used to mitigate Cr(III) from aqueous solutions and this method

C. Nikagolla  R. Chandrajith (&)  C. B. Dissanayake Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka e-mail: [email protected]; [email protected] R. Weerasooriya Department of Soil Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka C. B. Dissanayake Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka

is found to be simple, effective, economical and environmentally benign. Keywords Natural red earth  Chromium(III)  Surface complexation  Diffuse layer model (DLM)

Introduction Pollution of the environment by chromium waste is becoming widespread worldwide due to its wide range of industrial applications such as tanning, metallurgy (steel and non-ferrous alloys), electroplating, etc., and the situation is more severe in developing-countries where waste-treatment facilities are lacking. Therefore, contamination of environmental media such as soil and water by chromium containing wastes has become a matter of great concern in recent years. In the environment, chromium can occur in varying oxidation states from -2 to ?6 where the trivalent form [Cr(III)] and hexavalent form [Cr(VI)] are the most abundant (Palmer and Puls 1994). The hexavalent chromium is the most toxic and highly mobile form of chromium, which is considered as carcinogenic and lethal to animals and humans if ingested in large doses (Zayed and Terry 2003). According to the United States Environmental Protection Agency (USEPA), Cr(III) is also considered a hazardous substance which can cause several health impacts on humans. Cr(III) can also be readily oxidized to the Cr(VI) form (Calder 1988). It is, therefore, equally important to control Cr(III) in the environment to the accepted level of 0.1 mg/L for the industrial effluent discharges to the surface water and 0.05 mg/L of Cr(III) for potable water (EPA 1990). With increasing contamination of the environmental media by Cr species, many attempts have been made to remove them from the contaminated sites. Reduction (Kim et al. 2002), chemical precipitation (Ozer et al. 1997),

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adsorption (Lotfi and Adhoum 2002), solvent extraction (Mauri et al. 2001), freeze separation, reverse osmosis (Padilla and Tavani 1999), ion exchange (Rengaraj et al. 2003) and electrolytic-removal methods (Namasivayam and Ranganathan 1993) are the commonly applied techniques to treat different Cr species from contaminated media. The USEPA proposes coagulation/filtration, ion exchange resins, reverse osmosis and lime softening as the best technologies for chromium removal that involves expensive technology. Several attempts have been made to develop cost effective, simplified methods to remove Cr(III) from contaminated media using naturally available substances. Waste products from mining, industrial or agricultural operations may have potential of chromium removal. For instance, apricot stone, almond shells (Demirbasa et al. 2004), palm oil shells (Modrogan et al. 2007), rice husk (Srinivasan et al. 1988) and coconut husk (Tan et al. 1993) were investigated for their capabilities of removal of Cr(III) from aqueous media under different experimental conditions. Mineral substances such as pyrite (Zouboulis et al. 1995) and clino-pyrrhotite (Lu et al. 2006) were also used as natural low cost absorbents of Cr. Natural red earth (NRE) is a red-colored, sandy soil that naturally occurs abundantly in the northwest coastal belt of Sri Lanka underlain by Miocene limestone sequences. The affinity of NRE toward transition metals such as Ni and As have been studied in number of occasions (Peris 1998; Vithanage et al. 2005; 2007). The present study investigates the adsorption of Cr(III) onto NRE under selected physicochemical conditions. It will provide a conceptual frame work to remove Cr(III) from contaminated water. The generalized diffused-layer model (DLM) was employed to quantify Cr(III) adsorption by NRE specifically due to the following reasons: (a) the single-plane modeling approach eliminates the estimation of several fitting parameters associated with multi-layer models without sacrificing accuracy (Stumm and Morgan 1996); (b) the surface charge and potential relationship is determined by the Gouy Chapman theory; (c) the combination of high inner-layer capacitance and low outer layer capacitance of multi-layer models indicates that the two planes are embedded in a porous mineral surface layer of finite thickness; (d) the NRE is considered to contain two site types, namely[AlOH and [FeOH (Vithanage et al. 2005); (e) the generalized DLM is capable of handling several surface sites simultaneously. Hence, this model provides an excellent platform for the calculation of interfacial properties of NRE owing to its inherent simplicity.

Materials and methods Natural red earth samples used in the study were collected from the Aruwakkaru limestone quarry site in the

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Environ Earth Sci (2013) 68:641–645

northwestern part of Sri Lanka (latitudes and longitudes of 0 0 8°14 5000 N and 79°45 4500 E). It mainly consists of SiO2 (54.15 %), Al2O3 (20.73 %) and Fe3O2 (12.39 %), where SiO2 is present in crystalline form while Al (as Al2O3) and Fe (as Fe2O3) exist as an amorphous coating around the silica grains (Vithanage et al. 2005). It occurs as rounded and well-sorted quartz sand in a red clayey matrix with accessory ilmenite and magnetite. The brick red color of NRE indicates oxidizing conditions for the formation of red hematite. The NRE contains 0–1 % Fe2?and typically a higher ([2.0 %) Fe3? content, sometimes reaching up to 6 % (Dahanayake and Jayawardana 1979). Samples were air dried and the sieve fraction of 125–180 lm was used for the adsorption experiments. Preliminary studies were carried out to determine the optimum NRE concentration which behaves as a homogeneous system upon constant stirring. A series of solutions were prepared with the NRE concentrations, varying from 1.0 to 10.0 g/L and solutions were then subjected to constant stirring. At every 5-min interval, 10 ml aliquots were taken from the system to determine the amount of solid phase in the solution. Based on the preliminary study, 2 g/L was identified as the optimum NRE substrate concentration that behaves as a homogeneous system upon constant stirring; hence, this substrate concentration was used in all subsequent adsorption studies. Initial adsorption experiments were carried out to obtain the time required for the optimum adsorption using Cr(III) solutions with the concentrations of 0.038, 0.096 and 0.192 mM. These Cr(III) solutions with pH values of 2.0, 4.0, 6.0, 7.0, 8.0 and 10.0 were spiked with 2 g/L NRE and stirred constantly throughout the experiments. The pH adjustments of solutions were performed using 5 M NaOH and 1 M HNO3. The pH dependency of Cr(III) adsorption onto NRE was measured at pH ranges from 2.0 to 10.0 and the 0.096 mM of Cr(III) solution that were spiked with 2.0 g of the material. Every 5 min, using disposable syringes, 10.0 ml of aliquots were extracted from the systems during the experiments. The pH of the system was recorded throughout the adsorption experiment. The solid phase in the aliquots was separated immediately using a disposable cartridge filter with a 0.45-lm membrane. The supernatant was stored at 4 °C until the analysis was carried out using an atomic absorption spectrophotometer (AAS-Varian 240FS). The adsorption isotherm was constructed at pH 2 at 298 K. Series of Cr(III) solutions with concentrations of 0.0385, 0.096, 0.192 and 0.480 mM spiked with 2 g of NRE were subjected to constant stirring for 90 min. The initial and final pH measurements were recorded and the Langmuir adsorption isotherm was used to model the adsorption data. The experimental data were optimized and quantified using FITEQL computer algorithm (Herbelin and Westall 1999)

Environ Earth Sci (2013) 68:641–645

643 100

Table 1 Physico-chemical data of NRE and formation constants of various solute species used for the diffused layer model

90

(a)

Parameters

Value

Source

Surface area

350 m2/g

Vithanage et al. (2005)

pHZPC

8.8

Vithanage et al. (2007)

Equilibrium constants CrOH

2?

-4.00

Adsorption (%)

80

-9.62

Harrison (2007)

Cr(OH)3

-16.75

Harrison (2007)

60 50 40 0.1920 mmol/L Cr(III)

30

0.0962 mmol/L Cr(III)

20

Harrison (2007)

Cr(OH)? 2

70

10 0

0.0385 mmol/L Cr(III)

pH=2.00 0

10

20

30

40

50

60

70

80

90

100

Time (min) 100

Table 2 Optimized FITEQL results for Cr(III) adsorption onto natural red earth (NRE) Value

Site density AlOH

1.375 9 10-5

FeOH

1.673 9 10-4

(b)

80

Adsorption (%)

Parameter

90

70 60 50 40 30 20

log K

10

pH=4.00

0

AlOH AlOH2? AlOFeOH

0

10

20

30

40

7.495

50

60

70

80

90

100

Time (min)

-6.964 100

FeOH2

4.984

FeO-

-7.244

2AlOH(s) ? Cr ? (OH)? 2 (aq) ? (AlO)2Cr (s) ? 2H2O

15.19

2FeOH(s) ? Cr ? (OH)? 2 (aq) ? (FeO)2Cr (s) ? 2H2O

5.077

90

(c)

80

Adsorption (%)

?

70 60 50 40 30 20 pH=10.0

10

to determine the Cr(III) binding constant. Only [FeOH and [AlOH were assumed as adsorption sites. The experimental data were obtained for the Cr(III) concentration of 0.0385 mM. Table 1 shows the parameters used for the data modeling while Table 2 gives the optimized FITEQL results for Cr(III) adsorption onto NRE.

Results and discussion Cr(III) adsorption Variation in the Cr(III) adsorption onto NRE as a function of contact time (tC) at 298 K was examined over the pH range 2–10 for the concentrations of 0.192, 0.096 and 0.0385 mM. It was observed that the adsorption is a twostep process that terminated within 90 min. Initially, the adsorption increased rapidly where over 95 % of the Cr(III) in the solution was adsorbed onto NRE, and then, reached an apparent plateau within the first 10 min (Fig. 1). After reaching a constant plateau, minor variations in the adsorption values were observed which was probably due

0 0

10

20

30

40

50

60

70

80

90

100

Time (min)

Fig. 1 Cr(III) adsorption on to NRE as a function of time at different concentrations. Adsorptions at pH 6, 7 and 8 were same as the pH 4

to slow diffusion back to the solution. The large surface area of NRE causes this rapid adsorption step followed by slow diffusion. The diffusion could occur mainly along the micro pore walls of solids (Anderson et al. 1976); hence, no attempts were made to collect data over an extended period of contact time. Therefore, in all the subsequent experiments, the equilibration time (tC) of Cr(III)–NRE suspension system was restricted to 90 min. The smooth Cr(III) adsorption versus time (tC) curve shows that the system continuously leads to saturation, with a possible mono-layer Cr(III) coverage of the sorbent (Malek and Yusof 2007). pH dependency The adsorption of Cr(III) on NRE, was optimal (*100 %) over the pH range 4.0–9.0 at initial Cr(III) loading of

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Environ Earth Sci (2013) 68:641–645 - -X -X

100

X

- -X -X -X -X X- -X X- -X -X X- X- -X -X -X -X X-

Table 3 Langmuir isotherm constants for Cr(III) adsorption at pH 2

X

90

Adsorption (%)

Parameters

pH 10 9 - 8 X 6 4 3 2

80 70 60 50 40

Value

Monolayer capacity (Cmax/mol m-2) 3

8.82457 9 10-5

-1

Energy of adsorption (k/m mol )

65.12643678

R2

0.975

30 20 0 0

10

20

30

40

50

60

70

80

90

100

Time (min)

Fig. 2 Cr(III) (0.096 mM) adsorption onto NRE (2.000 g) as a function of pH values

0.096 mM (Fig. 2). However, with the decreasing pH, the adsorption also decreases which is more pronounced at high initial Cr(III) loadings. The optimal value of Cr(III) adsorption decreases to about 25 % when there is excessive loading of Cr(III) (0.192 mM) (Fig. 1). The decrease in Cr(III) adsorption at the low pH is caused by the competition between excess H? ions and Cr(III) ions for the adsorption sites. When pH is [10, NRE exhibits slightly low adsorption capacity (95 %). The solid shows a certain degree of dissolution as indicated by reddish cloudiness in the system that probably resulted in the decrease of Cr(III) adsorption. Therefore, adsorption studies have not been carried out at these pH levels. This also indicates that the Cr(III) retention efficiency depends on both initial loading and the pH of the solution. The adsorption density was optimal when pH is [3.0. Adsorption isotherm The adsorption density versus Cr(III) equilibrium concentration was constructed for the pH range of 2.0–10.0. The experimental results fit well with the Langmuir isotherm only at pH = 2, suggesting mono-layer coverage of Cr(III) on the homogenous surface (Fig. 3). When pH is [2, the 6.00x10

-5

-5

Γ [Cr(III)]

5.00x10

4.00x10-5 -5

3.00x10

adsorption shows a slight deviation from the Langmuir isotherm. However, NRE has two different adsorption sites (i.e., [AlOH and [FeOH) with different adsorption energies. On the other hand, due to the surface roughness, on an atomic scale, there will be a difference in reactivity among the sites with the same coordination (Weerasooriya et al. 2001). These combined effects generate a surface with a wide range of adsorption energies that can subsequently result in a heterogeneous surface. Accordingly, it was assumed that at least for the experimental conditions imposed in this study, the sites are homogeneous. Parameters obtained for the Langmuir isotherm are shown in Table 3. Data modeling The modeling of Cr(III) adsorption onto NRE was carried out with the generalized diffuse layer model. The [AlOH and [FeOH sites, were believed to form due to surfaces of amorphous Fe or Al coatings in NRE that were considered as active sites with different proton affinity constants (Vithanage et al. 2005). The densities of [AlOH and [FeOH site ratio were kept around *1:10 and assumed that Cr(III) forms bidentate complexes with both [AlOH and [FeOH sites on NRE. The following reaction stoichiometries were introduced to model the Cr(III) adsorption data. The variation of pH upon Cr(III) adsorption on NRE is

-2 Adsorption Density (mol m )

10

9.00x10-5 8.00x10

-5

7.00x10-5 -5

6.00x10

5.00x10-5 4.00x10-5 3.00x10-5 2.00x10-5

-5

2.00x10

0.0 0.1 0.2 0.3

1.00x10

-5

0.00x10

2

3

4

5

6

7

8

9

10

pH Fig. 3 Langmuir isotherm of Cr(III) adsorption on NRE at pH 2

123

0.4 0.5

0.6

[Cr(III)]eq mol/L

-5

Fig. 4 The adsorption density of NRE as a function of equilibrium pH with initial Cr(III) concentration of 0.0385 mM. The Cr(III) adsorption data well interpreted with DLM model at pH 2. When pH is changed, an apparent desorption was observed due to site heterogeneity

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not marked; therefore, the following reaction stoichiometries are suggested: þ 2 [ AlOHðsÞ þ Cr ðOHÞþ 2 ðaqÞ ! ð[AlOÞ2 CrðsÞ þ 2H2 O

ð1Þ 2 [ FeOHðsÞ þ

CrðOHÞþ 2 ðaqÞ

! ð[FeOÞ2 Crþ ðsÞ þ 2H2 O ð2Þ

The model calibrations were carried out using the experimental data obtained for the initial Cr(III) concentration of 0.0385 mM (Fig. 4). The mass action coefficient of the reaction (1) and (2) were obtained by optimization. The model data fit well with experimental values and the optimized log K values are given in Table 2.

Conclusions Almost 100 % of Cr(III) in solution adsorb readily onto NRE over the pH range 4.0–9.0 and at the solid concentration of 2 g/L. The adsorption was the one which essentially completes within 90 min. The adsorption isotherm fits well with the Langmuir model at pH 2, suggesting the presence of homogeneous surface and the occurrence of mono-layer adsorption. Cr(III) also showed strong affinity for NRE forming an inner-sphere complex over the two surface sites, [FeOH and [AlOH. It is evident that NRE can be used as a potent adsorbent to remove Cr(III) from contaminated water. Although the pHzpc of the NRE is 8.8, the affinity to NRE sites is very strong; thus, it may override the potential field created due to charge variations. Further investigations are necessary to study the adsorption behavior of NRE and the validations of the model in the presence of competitive metal cations. Acknowledgments R.C. and C.B.D. gratefully acknowledged the Alexander von Humboldt (AvH) Foundation, Germany for the donation of a Varian 240FS Atomic Absorption Spectrophotometer used in this work.

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