Attenuation of divalent toxic metal ions using natural sericitic pyrophyllite

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Journal of Environmental Management 88 (2008) 1273–1279 www.elsevier.com/locate/jenvman

Attenuation of divalent toxic metal ions using natural sericitic pyrophyllite Murari Prasada,, Sona Saxenab a

Environment Chemistry Division, Advanced Materials and Processes, Research Institute (C.S.I.R.), Hoshangabad Road, Bhopal 462 026, India b Environment Division, Spatial Decisions, New Delhi 110048, India Received 26 July 2006; received in revised form 10 May 2007; accepted 2 July 2007 Available online 30 August 2007

Abstract The present study investigated the effectiveness of an inexpensive and ecofriendly alumino silicate clay mineral, sericitic pyrophyllite, as an adsorbent for the possible application in the removal of some divalent toxic metal cations such as Pb2+, Cu2+and Zn2+ from aqueous systems. Batch scale equilibrium adsorption studies were carried out for a wide range of initial concentration from 24.1 to 2410 mmol L1 for lead, 78.65 to 7865 mmol L1 for copper and 76.45 to 7645 mmol L1 for zinc solutions. The removal of Pb2+ was almost complete at low concentration (maximum lead removal capacity, LRC, 32 mg of lead/g of pyrophyllite) with 10 g L1 of adsorbent in a 30 min equilibration time. The effects of temperature on adsorption of heavy metal ions were studied. The applicability of the Langmuir, Freundlich and Dubinin–Radushkevich adsorption models in each case of lead, copper and zinc adsorption was examined separately at different temperatures. The adsorption process was found to be endothermic and the Freundlich adsorption model was found to represent the data at different temperatures more suitably. r 2007 Elsevier Ltd. All rights reserved. Keywords: Attenuation; Divalent toxic metals; Adsorption; Sericitic pyrophyllite

1. Introduction Metal ions’ presence in the environment is posing a serious threat to living species due to their toxicity. Unlike organic pollutants, the metal ions do not degrade into harmless end products. Therefore, the elimination of heavy metals from waters and wastewaters is of major concern to protect public health. The metal of immediate concern is lead, copper and zinc (Potgieter et al., 2006). Lead is ubiquitous in the environment and hazardous at high levels. Process industries, such as battery manufacturing and metal plating and finishing, are prime sources of lead pollution. Mining industries are also one of the sources of lead, copper and zinc metal pollution. Precipitation, ion exchange, solvent extraction, phytoextraction, ultrafiltration, reverse osmosis, electrodialysis and adsorption onto Corresponding author. Tel.: +91 0755 2488522; fax: +91 0755 2587042. E-mail addresses: [email protected], [email protected] (M. Prasad).

0301-4797/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2007.07.013

activated carbon are the conventional methods for removal of heavy metal ions from aqueous solutions (Kahashi and Imai, 1983; Huang and Blakenship, 1984; Applegate, 1984; Sengupta and Clifford, 1986; Gonzales-Davila et al., 1990; Geselbarcht, 1996). Till date, the only sorbent typically used for low concentration of heavy metal ions is active carbon which separates by adsorbing relatively low amounts of metal ions and is very unselective to the type of metal under consideration. Numerous approaches have been studied for the development of cheaper metal sorbents, such as fly ash (Ricou et al., 2001), peat (Brown et al., 2000; Ho and McKay, 2000), clays and related minerals (Huang and Hao, 1989; Daza et al., 1991; Zamzow et al., 1990), phosphate minerals (Nriagu 1972, 1973), etc. Synthetic hydroxyapatite has been used in wastewater treatment and has a very high capacity for removing divalent heavy metal ions from water (Ma et al., 1994; Yuping, and Schwartz, 1994). High grade (430% P2O5) rock phosphates have also been utilized (Ma et al., 1995) for the removal of different heavy metal ions from aqueous solutions and have been found to

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be effective in the separation of lead ranging from 38.8% to 100% depending on its initial concentration. In recent years clays and its related minerals are highly valued for their adsorptive properties. They have been explored as pollution sorbents for heavy metals (Huang and Hao, 1989; Sikalidis et al., 1989; Daza et al., 1991). Hydroxyapatite is too costly to be commercially used. However, sericitic pyrophyllite is relatively much inexpensive mineral abundantly found in Madhya Pradesh (India) and need to be explored for application as adsorbents in separation of heavy metal ions. Accordingly, the present investigation was undertaken to examine the utility of pyrophyllite for the removal of divalent cations from aqueous solutions. Sericitic pyrophyllite, due to its physical and chemical properties, i.e., specific surface area, cation exchange capacity and adsorption affinity for organic and inorganic ions, is considered as one of the most promising adsorbent for use in decontamination of high-level heavy metal wastes. Pyrophyllite is abundantly found alumino silicate minerals that have adsorbent properties due to their large surface areas and negative layer charge when in contact with water (Gucek et al., 2005). The chemical species, in general, can interact with clay by two-dimensional, i.e., physical (ion exchange/physiosorption) and chemical (surface precipitation) adsorption process. On clay minerals such as pyrophyllite and montmorillonite, adsorption can occur via two different mechanism: (i) cation exchange in the interlayers resulting from the interactions between ions and negative permanent charge and (ii) formation of inner-sphere metal complexes through Si–O and Al–O groups at the both edge sites of clay particle (Schindler et al., 1976; Mercier and Detellier, 1995; Kraepiel et al., 1999). Recent studies using surface spectroscopic and microscopic techniques have shown that in many cases the sorption of heavy metals on clay surfaces results in the formation of three-dimensional multinuclear or polynuclear surface phases (Scheidegger et al., 1997, 1998; Elzinga and Sparks, 1999; Fendorf and Sparks, 1994). The objective of the present studies described here was to investigate sorption characteristics of Pb2+, Cu2+ and Zn2+ onto sericitic pyrophyllite. Isotherm studies for adsorption of Pb2+, Cu2+ and Zn2+ onto pyrophyllite have been investigated in details and reported. 2. Materials and methods 2.1. Materials Representative samples of high alkali sericitic pyrophyllite were procured from Jhansi, Uttar Pradesh, India. The sample (1.0 kg) was ground in ball mill and sized by wet sieve analysis separately for experimental work. The representative ground sample after coning and quartering was subjected to chemical analysis. The chemical analysis is given in Table 1. Based on sorption characteristics, the

Table 1 Physical properties and chemical composition of sericitic pyrophyllite clay sample Physical properties

Surface area: 1.45 m2 g1 C.E.C.: 1.238 meq g1 Porosity: 45.2% Specific gravity: 2.6489 Hardness: 1.35 a

Chemical composition Constituents

Weight percent

SiO2 Al2O3 Fe2O3 TiO2 MgO CaO K2O Na2O L.O.I.a

53.00 28.14 0.61 0.10 1.09 1.63 9.21 0.32 5.55

L.O.I., loss on ignition.

mineral sample of 75+53 mm size range was used for adsorption studies. Characterization studies of sericitic pyrophyllite carried out by X-ray diffraction, I.R. and S.E.M., reported mainly of pyrophyllite, a-quartz, kaolinite and muscovite (Saxena, 2001). Due to the presence of muscovite phase, the pyrophyllite sample used in the present studies is termed as ‘sericitic pyrophyllite’.

2.2. Methods Stock solutions were prepared by dissolving separately nitrate salts of Pb2+, Cu2+ and Zn2+ obtained from Merck using double distilled water. All metal ions were serially diluted to prepare solutions of varying initial concentration for experimental work. For all adsorption tests, blank experiments were carried out with the same experimental procedure to check the extent of adsorption by the glass flasks. The adsorption experiments were performed in different batches. One-gram adsorbent clay mineral samples were equilibrated separately with 100 ml solutions of different concentrations of lead, copper and zinc under natural conditions, i.e., without imposition of any ionic strength. The suspensions were shaken on a mechanical shaker at room temperature for 10 min followed by 30 min quiescent contact time. Suspensions were then filtered through Whatman filter paper (no. 42). The filtrates were analyzed for Pb2+, Cu2+ and Zn2+ concentration using Atomic Absorption Spectrophotometer (GBC make model no. 902). For arriving at the required quiescent time needed for attainment of adsorption equilibrium, the quiescent time was varied between 10 and 40 min at intervals of 5 min for different initial lead, copper and zinc ion concentration. The equilibrium was observed to reach within 30 min time. Random tests were done for different concentration to check the reproducibility of the results.

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3. Results and discussion 3.1. Effect of initial pH The influence of pH on the adsorption of divalent metal ions on sericitic pyrophyllite was examined in the pH region between 1.0 and 7.0 as depicted in Fig. 1. The pH was limited to 7.0 because of precipitation of metal hydroxide at higher pH as described elsewhere (Elzinga and Sparks, 1999; Potgieter et al., 2006; Sengupta and Bhattacharya, 2006; Erdemoglu et al., 2004). The experiments were run in triplicate. The deviation in the results was of the order of 71.2% with a reproducibility of 70.5%. It can be observed from the Fig. 1 that the adsorption of Pb2+, Cu2+ and Zn2+ increases with pH and reaches a maximum at pH 3.0 for all the three metal ions. At pH 43.0, percent adsorption decreases slightly till pH ¼ 6.0 and thereafter it appears to remain constant. As the pH changes, surface charge also changes and the sorption of charged species is affected (attraction between the positively charged metal ions and the negatively charged clay surface). It is conceivable that at low pH, where there is an excess of H3O+ ions in solution, a competition exists between the positively charged hydrogen ions and metal ions for the available adsorption sites on the negatively charged pyrophyllite surface. As the pH increases and the balance between H3O+ and OH is more equal, more of the positively charged metal ions in solutions are adsorbed on the negative clay surface and thus the percentage removal of metal ions increases (Potgieter et al., 2006). 3.2. Effect of temperature The percent adsorption of divalent metal cations (Pb2+, Cu2+ and Zn2+) increases progressively with the gradual increase in temperature from 30 to 60 1C, as shown in

Fig. 2. Effect of temperature on sorption of divalent metal ions (adsorbent mass ¼ 10 g L1, volume ¼ 100 ml, equilibration time30 min, metal concentration ¼ 30–100 mg L1.

Fig. 1. Effect of pH on adsorption of divalent metal ions (adsorbent mass ¼ 10 g L1, volume ¼ 100 ml, equilibration time ¼ 30 min, metal concentration ¼ 20 mg L1).

Fig. 2. Experiments at lower metal concentrations (1.44  104 to 4.83  104 M for Pb2+, 4.72  104 to 15.74  104 M for Cu2+ and 4.58  104 to 15.3  104 M for Zn2+) demonstrated the maximum (about 85–100% for Pb2+ and 75–95% for Cu2+ and Zn2+) removal of all the three divalent cations. It indicates the sorption process on pyrophyllite clay to be endothermic in nature. However, the range of increase of percent adsorption of lead is greater than that of copper and zinc. The adsorption

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isotherms at higher concentrations are regular, positive and concave to the axis. The plot for Pb2+ adsorption (Fig. 2) shows that lead is nearly 100% adsorbed at lower concentration (p1.5  104 M). Similarly the adsorption of copper and zinc is nearly 95% up to concentration p5.0  104 M (Fig. 2). Almost complete removal at low concentration of all the three metal ions and the steep increase in early stages of the isotherms indicate a faster initial removal. 3.3. Sorption isotherms Adsorption results out of a mass transfer process occurring at an interface between solid and liquid phases. Equilibrium relationships between adsorbent and adsorbate are described by sorption isotherms usually as the ratio between the quantity sorbed and that remaining in the solution at a fixed temperature. Experimental isotherm data are often described and modeled by the relations developed by Freundlich, Langmuir and Dubinin–Radushkevich. 3.3.1. Langmuir isotherm The Langmuir model can be expressed as: qe ¼

Q0 bC e 1 þ bC e

(1)

A linear form of this equation is Ce 1 Ce þ ¼ Q0 b Q0 qe

(2)

where qe (mmol g1) is the amount of metal ions adsorbed on pyrophyllite at equilibrium, Ce (mmol L1) is the equilibrium concentration of metal ions, Q0 and b are Langmuir constants related to maximum sorption capacity and sorption energy, respectively. The equilibrium data for divalent metal cations over the concentration range 30–100 mg L1 have been correlated separately at different temperatures (303, 313, 323 and 333 K) with the Langmuir isotherms (Fig. 3). A linear plot is obtained at all temperatures for the three cations when Ce/qe is plotted against Ce over the entire concentration range of metal ions investigated. The Langmuir model parameters and the statistical fits of the sorption data to this equation are given in Table 2. The Langmuir model effectively described all the sorption data with all R2 values 40.98 indicating monolayer adsorption (see Table 3). The value of Q0 (maximum uptake) increases (188.6–370 mmol g1 for Pb2+, 588–833 mmol g1 for Cu2+, 588.2–714 mmol g1 for Zn2+) with increase in temperature from 303 to 333 K for all the three cations, thereby indicating the endothermic nature of the adsorption process. The sorption energy, b, increases with the temperature (303–333 K) for all the cations under consideration and is found to be 6.56  105 to 8.8  105, 5.85  105 to 7.6  105 and 3.14  105 to 5.6 

Fig. 3. Langmuir sorption isotherm of divalent metal cations on sericitic pyrophyllite at different temperatures (absorbent mass ¼ 10 g L1, pH ¼ 8.5, V ¼ 100 ml, equilibration time t ¼ 30 min).

105 L mol1, respectively, for Pb2+, Cu2+ and Zn2+. These values of b suggest that sorption on pyrophyllite follows the following preferential sequence: Pb2+4Cu2+4Zn2+. 3.3.2. Freundlich isotherm The Freundlich model is described by the following equation: qe ¼ K F C 1=n e

(3)

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Table 2 Langmuir isotherm constants of metal ion sorption on sericitic pyrophyllite Temperature (K)

303 313 323 333

Pb2+

Cu2+

Zn2+

Qo (mmol g1)

b  105 L mol1

R2

Qo (mmol g1)

b  105 L mol1

R2

Qo (mmol g1)

b  105 L mol1

R2

188.6 303.0 333.0 370.0

6.56 7.36 8.12 8.8

0.999 0.989 0.998 0.991

588 625 769 833

5.85 6.18 6.86 7.6

0.996 0.996 0.998 0.992

588.2 625.0 666.6 714.3

3.14 4.0 4.59 5.6

0.996 0.991 0.976 0.979

Table 3 Freundlich isotherm parameters Temperature (K)

303 313 323 333

Pb2+

Cu2+

Zn2+

1/n

KF (mmol g1)

R2

1/n

KF (mmol g1)

R2

1/n

KF (mmol g1)

R2

0.427 0.463 0.470 0.495

15.4 17.0 19.4 22.1

0.956 0.957 0.966 0.970

0.371 0.379 0.391 0.421

34.2 41.1 46.9 47.86

0.992 0.960 0.963 0.931

0.337 0.343 0.345 0.356

38.3 42.9 47.5 52.2

0.989 0.986 0.974 0.997

The above equation may be rewritten as: 1 log qe ¼ log K F þ log C e n

(4)

Both KF and n are empirical constants being indicative of the extent of adsorption (sorption capacity) and the degree of nonlinearity between solution and concentration. The experimental sorption data are well described by Freundlich model at all temperatures and the results are shown in Fig. 4. All the values of Freundlich parameters were calculated from the slope and intercept values of straight lines and tabulated in Table 4. The adsorption capacity, KF, has an increasing trend (15.4–22.1 mmol g1 for Pb2+, 34.2–47.86 mmol g1 for Cu2+ and 38.3–52.2 mmol g1 for Zn2+) with temperatures, indicating that adsorption capacity is increased at higher temperature and the adsorption process is of endothermic nature. The numerical values of 1/n are less than unity for lead, copper and zinc at all temperatures, indicating favorable adsorption (Hasany et al., 2002). 1/n has comparatively higher values for lead than both copper and zinc at all temperatures. 3.3.3. Dubinin–Radushkevich (D–R) isotherm The sorption data have been analyzed by D–R model which is expressed in the following form: ln C ads ¼ ln X m  b2

(5)

where Cads is the amount of divalent metal ions adsorbed on per unit weight of adsorbent (mg g1), Xm is the maximum sorption capacity, b is the activity coefficient and e is the Polanyi potential equaling to   1  ¼ RT ln 1 þ (6) Ce

where R is the universal gas constant (KJ mol1 K1) and T is the temperature (K). The slope of the straight line plot ln Cads versus e2 gives b (mol2 J2) and the intercept yields the sorption capacity Xm (mmol g1). The mean sorption energy (E) is a critical parameter for distinguishing between physical and chemical adsorption, for heterogeneous surface of the adsorbent and it is given by 1 E ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi . ð2bÞ

(7)

The plot of ln Cads versus e2 has been shown in Fig. 5. The D–R parameters have been calculated for the sericitic pyrophyllite from the slope and intercept of the straight line shown in Fig. 5 and presented in Table 4. The E-values are 4.56 kJ mol1 for Pb2+, 3.66 kJ mol1 for Cu2+ and 2.535 kJ mol1 for Zn2+ on the natural pyrophyllite sample that has heterogeneous structure. The numerical value of E is in the ranges of 1.0–8.0 and 9.0–16.0 kJ mol1 for physical and chemical adsorption respectively (Saeed, 2003). The maximum sorption capacity, Xm, using the D–R equation was found to be 17.11 mmol g1 for Pb2+, 54.76 mmol g1 for Cu2+ and 54.86 mmol g1 for Zn2+. 3.4. Mechanism of removal of divalent cations by sericitic pyrophyllite A number of governing mechanisms such as surface adsorption, dissolution and subsequent precipitation (complexation), ion exchange, absorption, etc., have been reported for the separation of aqueous heavy metals, namely, Pb, Cu, Zn, Cd, etc., by many investigators. Processes such as adsorbent dissolution, cation exchange, metal complexation on adsorbent (apatite/rock phosphate) surface, followed by a new metal phase precipitation, i.e.,

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Fig. 5. D–R sorption isotherm for divalent cations by sericitic pyrophyllite (absorbent mass ¼ 10 g L1, T ¼ 3030 K, V ¼ 100 ml, equilibration time t ¼ 30 min).

Fig. 4. Freundlich sorption isotherm of divalent metal ion on sericitic pyrophyllite at different temperatures (absorbent mass ¼ 10 g L1, pH ¼ 8.5, V ¼ 100 ml, equilibration time t ¼ 30 min).

Table 4 D–R isotherm parameters D–R parameters

Pb2+

Cu2+

Zn2+

Xm (mmol g1) 17.11 54.76 54.86 b (mol2 J2) 24.1  109 37.2  109 77.8  109 Sorption energy E (kJ mol1) 4.56 3.66 2.535 Correlation coefficient, R2 0.964 0.971 0.9805

chemisorption have been proposed as a function of concentration. The values of mean sorption energy obtained in the present studies suggest removal of divalent cations by physical adsorption. This observation is supported by the fact that both physical and chemical

adsorption processes are predominant as a function of the concentration range used (Prasad and Saxena, 2004). The present studies were carried out for a concentration range 10–100 mg L1 of divalent cations. At low concentration (o100 mg L1) essentially physical adsorption without excluding ion exchange becomes preponderant and a remarkable deviation occurs at higher concentration (4100 mg L1). This could probably be due to the formation of a complex phenomenon such as mass transfer by means of chemical reactions (Chegrouche et al., 1997). One possible explanation of physical adsorption at lower concentration may be the release of weakly bonded potassium ions at higher temperature into the solution that makes it easy for metal ion removal by ion exchange. The dominance of muscovite [KAl2(AlSi3O10)(F,OH)2] which has a layered structure of aluminum silicate sheets weakly bonded together by layers of potassium ions is observed chemical analysis (Table 1) of sericitic pyrophyllite. The weakly bonded potassium ions may get released into the solution at higher temperature and cation exchange may occur in the interlayers resulting from the interactions between cations and negative permanent charge (Schindler et al., 1976; Mercier and Detellier, 1995; Kraepiel et al., 1999). This assumption is well supported by the occurrence of only ion exchange with increasing temperature observed during removal of lanthalum by bentonite clay (Chegrouche et al., 1997; Donat et al., 2005). It is for the reader’s kind information that the problem of disposal of used adsorbents was solved by converting them into a value-added product. 4. Conclusions The objective of the present investigation was to study the effectiveness of sericitic pyrphyllite as an adsorbent for

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the removal of divalent metal ions (Pb2+, Cu2+ and Zn2+) from aqueous solutions. (i) The metal uptake affinity order was found: Pb2+4Cu2+4Zn2+. (ii) Almost complete removal of all the three divalent cations at lower metal concentrations (1.44  104 to 4.83  104 M for Pb2+, 4.72  104 to 15.74  104 M for Cu2+ and 4.58  104 to 15.3  104 M for Zn2+) was demonstrated (about 85–100% for Pb2+ and 75–95% for Cu2+ and Zn2+). (iii) Sorption process on pyrophyllite clay is observed to be endothermic in nature.

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