Biosorption of Cu(II), Pb(II) and Zn(II) Ions from Aqueous Solutions Using Selected Waste Materials: Adsorption and Characterisation Studies

Wiwid Pranata Putra, Azlan Kamari, Siti Najiah Mohd Yusoff, Che Fauziah Ishak, Azmi Mohamed, Norhayati Hashim, Illyas Md Isa

Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tg. Malim, Perak, Malaysia.
Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tg. Malim, Perak, Malaysia; Nanotechnology Research Centre, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tg. Malim, Perak, Malaysia.
Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia, Serdang, Selangor, Malaysia.

DOI: 10.4236/jeas.2014.41004                

Abstract

The efficacy of coconut tree sawdust (CTS), eggshell (ES) and sugarcane bagasse (SB) as alternative low-cost biosorbents for the removal of Cu(II), Pb(II) and Zn(II) ions from aqueous solutions was investigated. Batch adsorption studies were carried out to evaluate the effects of solution pH and initial metal concentration on adsorption capacity. The optimum biosorption condition was found at pH 6.0, 0.1 g biomass dosage and at 90 min equilibrium time. The adsorption data were fitted to the Freundlich and Langmuir isotherm models. The adsorption capacity and affinity of CTS, ES and SB were evaluated. The Freundlich constant (n) and separation factor (R_L) values suggest that the metal ions were favourably adsorbed onto biosorbents. The maximum adsorption capacities (Q) estimated from the Langmuir isotherm model for Cu(II), Pb(II) and Zn(II) were 3.89, 25.00 and 23.81 mg/g for CTS, 34.48, 90.90 and 35.71 mg/g for ES, and 3.65, 21.28 and 40.00 mg/g for SB, respectively. The characterisation studies were performed using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectrometer (EDX) and Fourier Transform Infrared Spectrometer (FTIR). Interaction with metal ions led to the formation of discrete aggregates on the biosorbents surface. The metal ions bound to the active sites of the biosorbents through either electrostatic attraction or complexation mechanism.

Keywords

Adsorption; Characterisation; Metal Ions; Coconut Tree Sawdust; Eggshell; Sugarcane Bagasse

1. Introduction

Excessive levels of toxic metals in the water environment have been a worldwide issue for many years [1] . Heavy metals are introduced into water by several industries such as mining, electroplating, petroleum refining and etc. They may pose toxicity to human especially at high concentration [2] -[4] . However, due to their high industrial value their application in industries is irreplaceable. Therefore, it is very important to remove heavy metals from water environment [5] [6] .

Various techniques have been applied for the removal of heavy metals from water. This includes membrane filtration, ion exchange and chemical precipitation [7] . These techniques are costly. Adsorption has been proposed as a cost-effective method for water decontamination. Activated carbon is widely used as an adsorbent for water treatment. It is effective to sequester metal ions from water environment. However, activated carbon is expensive [8] [9] .

In recent years, a number of alternative adsorbents have been studied for water clean-up. They are inexpensive, efficient and practical to be utilised. Agricultural and industrial wastes, as well as natural minerals are widely used as alternative biosorbents for many years [1] [5] . Agricultural wastes such as tea waste [10] [11] , coffee waste [10] , watermelon seed hulls [12] , kapok fibre [13] , lam tree (Cordia africana) sawdust [14] , Ricinus communis [15] and coir fibre [16] have been tested for metal ion adsorption. The application of agricultural wastes as biosorbents for the removal of heavy metals has many advantages such as available in large quantities, renewable in nature, eco-friendly and low-cost [17] [18] .

Agricultural waste normally contains a variety of organic compounds (lignin, cellulose and hemicelluloses) and functional groups (hydroxyl, carbonyl and amino). Both organic compounds and functional groups have great affinity for metal ion complexation [11] -[13] . Coconut tree sawdust (CTS) and sugarcane bagasse (SB) are plant-based materials. They may contain a number of organic compounds and functional groups, meanwhile eggshell (ES) consists of carbonate that favours metal ion binding [19] .

The aim of this work was to evaluate the potential of CTS, ES and SB as alternative low-cost biosorbents for decontamination of metal ions from aqueous solutions. The adsorption studies were carried out as a function of solution pH and initial metal concentration. The equilibrium data were described by the Freundlich and Langmuir isotherm models. SEM, EDX and FTIR analyses were performed to elucidate the adsorption mechanism(s).

2. Materials and Methods

2.1. Preparation of Biosorbents and Solutions

CTS was supplied by Jati Cemerlang Sawmill, Selangor, meanwhile ES and SB were obtained from My Rasa Restaurant, Selangor. The materials were washed and dried in an oven at 70˚C for 7 days. The dried materials were ground using a laboratory mill and sieved through 150 - 250 µm size fraction using an American Society for Testing and Materials (ASTM) standard sieve.

Metal stock solutions (Cu(II), Pb(II) and Zn(II)) of 1000 mg/L were prepared by dissolving an appropriate amount of Cu(NO₃)₂∙3H₂O, Pb(NO₃)₂, and Zn(NO₃)₂ salts in 0.1 mol/L KNO₃. In this study, 0.1 mol/L KNO₃ was used as an electrolyte to control the ionic strength of metal ions. The stock solutions were diluted to the required concentrations using 0.1 mol/L KNO₃.

2.2. Characterisation Studies

The surface area and pore diameter of the biosorbents were determined using a Quantachrome Autosorb l Micromeritics Surface Analyser. The surface area was measured according to Brunauer-Emmett-Teller (BET) multipoint technique [20] , meanwhile the pore diameter was calculated based on Barrett, Joyner and Halenda (BJH) method [21] . The surface area of CTS, ES and SB were 0.4, 6.5 and 2.8 m²/g, respectively. Meanwhile, the pore diameter of CTS, ES and SB were 1.9, 11.4 and 7.2 nm, respectively. The surface morphology of the biosorbents was observed using a Hitachi SU 8020 UHR FESEM. The samples were coated with platinum to avoid charging. The elemental composition of the biosorbents was determined by using a Horiba Energy Dispersive X-ray Spectrometer. The functional groups of the biosorbents, as well as binding mechanism(s) were confirmed using a Thermo Nicolet 6700 Fourier Transform Infrared Spectrometer.

2.3. Batch Adsorption Studies

Batch adsorption experiments were carried out by adding 0.1 g of biosorbent to 25 mL of metal ion solution in a conical flask. The mixture was shaken for 90 min at 100 rpm using a protech orbital shaker (model 720). The optimum solution pH was determined in the pH range of 2.0 - 6.0. The solution pH was adjusted using 0.5 mol/L HCl or 0.5 mol/L NaOH. The solution pH before and after interaction with single metal ion was determined using a Thermo Scientific Orion 2-Star pH meter. The effect of initial metal concentration was studied in the range of 10 - 200 mg/L. However, adsorption of Pb(II) by ES was studied in the initial metal concentration ranging from 10 to 500 mg/L. The concentration of metal ion in the supernatant was measured using an AAnalyst 400 Perkin Elmer Atomic Absorption Spectrometer. The amount of metal ion adsorbed onto biosorbent was calculated using Equation (1)

(1)

where q is the amount of metal ion adsorbed, C_o is the initial metal ion concentration, C_e is the equilibrium metal ion concentration, w is the weight of biosorbent, and V is the volume of metal ion solution. Triplicate experiments were carried out for adsorption study.

3. Results and Discussion

3.1. Adsorption Studies

3.1.1. Effect of Solution pH

The pH of the solution significantly affects the amount of metal ion adsorbed onto biosorbents as it influences the properties of the biosorbents, as well as the speciation of metal ions in aqueous solution.

The effect of solution pH on metal ion adsorption is shown in Figure 1. From Figure 1, an increase in the solution pH from 2.0 to 6.0 has increased the amount of Cu(II), Pb(II) and Zn(II) adsorbed. For example, the amount of Pb(II) adsorbed by CTS, ES and SB increased from 5.37 to 15.07, 16.87 to 24.70, and 5.71 to 13.05 mg/g, respectively. From Figure 1, the maximum adsorption of metal ions onto biosorbents was found at pH 6.0. Therefore, pH 6.0 was chosen as the optimum pH for the adsorption system. Many adsorption studies report pH 5.0 - 6.0 as the optimum pH for Cu(II), Pb(II) and Zn(II) adsorption by various biosorbents [22] -[24] .

At low pH (2.0 - 3.0), more H₃O⁺ ions will be available to compete with Cu(II), Pb(II) and Zn(II) ions for the adsorption sites of the biosorbents. In addition, at low pH most of the functional groups are protonated [25] . This will reduce the number of binding sites available for the adsorption of metal ions. The increase in adsorption capacity at higher pH values can be attributed to the weak inhibitory effect of H₃O⁺ ions. The solution pH was kept within the pH range of 2.0 - 6.0 because the precipitation of metal ion was occurred simultaneously at pH values higher than 6.0 [26] [27] .

3.1.2. Effect of Initial Metal Concentration

The effects of initial metal concentration on the adsorption of Cu(II), Pb(II) and Zn(II) ions onto biosorbents are shown in Figure 2. From Figure 2, the amount of metal ion adsorbed by CTS, ES and SB increased with increasing initial metal concentration. For example, the amount of Cu(II), Pb(II) and Zn(II) adsorbed onto ES increased from 2.33 to 30.66 mg/g, 2.42 to 83.35 mg/g, and 2.02 to 24.07 mg/g, respectively. A significant amount of metal ions adsorbed at high initial metal concentration can be related to two main factors, namely high probability of collision between metal ions with the biosorbent surface and high rate of metal ions diffusion onto biosorbent surface. As discussed by Wang et al. [28] , high initial metal concentration accelerates the driving force and reduces the mass transfer resistance.

Figure 1. Effect of solution pH on adsorption by (a) CTS, (b) ES and (c) SB.

Conflicts of Interest

The authors declare no conflicts of interest.

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