Kinetic and thermodynamic study of cadmium (Cd) adsorption by grape and apple pruning residues biochars

Abstract
In order to study the equilibrium, kinetic and thermodynamic of cadmium (Cd) adsorption by grape and apple pruning residues biochars, batch experiments carried out with different initial concentration of Cd (0 to 200 mg/L) with 0.03 M NaNO3 as a background solution. The effect of pH (4, 5, 6), ionic strengths (0.01, 0.03, 0.1 M) and temperature (10, 20, 30, 40 0C) at different times (0 to 240 minutes) were investigated. The results showed that the removal efficiency and adsorption capacity of adsorbents decreased with increasing initial concentration. Among adsorption kinetic models, pseudo second order model was better fitted for experimental data (R2=1) and the equilibrium time is attained in 40 min for gape and apple pruning residue biochars. Ionic strength and pH of background solution significantly affected Cd adsorption and the highest adsorption capacity was obtained at pH 6, ionic strengths of 0.01M at 40 0C. Sorption capacity factors (qmax, KF, KT, qD) and sorption energy factors (n, KL, B) of gape pruning residue biochar was more than apple pruning residue biochar. The sorption energy parameter (E) of Dubinin-Radushkevich isotherm and Gibbs free energy change (ΔG) indicated that the Cd adsorption processes were physical and spontaneous. The separation factor of Langmuir (RL) indicated that the sorption reactions of Cd2+ by grape and apple biochar is favorable (RL=0.15-0.50).

Keywords: Gape and apple pruning residue biochars, Cadmium, Adsorption isotherm, Adsorption kinetics, Adsorption thermodynamic.

Introduction
Cadmium (Cd) is a toxic heavy metal that has been released to the environment through the combustion of fossil fuels, metal production, application of phosphate fertilizers, electroplating, and the manufacturing of batteries, pigments, and screens. Given pervasive cadmium contamination and the low drinking water guideline, there is considerable interest in the development of techniques to remove cadmium from contaminated water. Several treatment processes such as adsorption, chemical precipitation, ion exchange and membrane filtration have been developed to remove the heavy metals dissolved in industrial wastewaters. Adsorption has been developed as a simple and effective technique for the removal of heavy metals from contaminated water and soil (Ahmaruzzaman 2010). Biomass is a renewable energy resource and has a growing interest as a chemical feedstock source. Biochar is a fine-grained carbonaceous solid residue, produced by pyrolysis of carbon-rich biomass under oxygen-limited conditions. Biomass-derived biochar is considered as a new potential, low-cost and effective adsorbent for heavy metal adsorbent, due to abundance of polar functional groups, such as carboxylic, hydroxyl, and amino groups, which are available for heavy metal removal (Sukiran et al., 2011). Biochar has many properties, such as a relatively structured carbon matrix, high degree of microporosity, extensive surface area, and high pH and cation exchange capacity (CEC); therefore, it may act as a surface adsorbent (Zhang et al. 2014). In the past few years, many reports on adsorption of various contaminants on biochar have been published including Peat moss biochar (Lee et al. 2015), sugarcane bagasse biochar (Yang et al. 2011), rice straw biochar (Han et al. 2013), sugarcane pulp residue biochar (Yang et al. 2014) and almond shells biochar (Kılıc et al. 2013). Agricultural residues, especially grape and apple pruning residues, being produced in large quantities in the vinyards and founding ways of using such residues for the treatment of water by producing biochar is necessary. Therefore, the objective of this research is to investigate the equilibrium, kinetic and thermodynamic of cadmium (Cd) adsorption by grape and apple pruning residues biochars from aqueous solution.

Materials and Methods
Preparation of Grape and apple pruning residue
Grape and apple pruning residue used in this study were obtained from orchards located in the west Azarbaijan province, Iran. The small pieces of grape and apple pruning residue after drying in the oven at 105°C for 24h, were placed in a vertical stainless steel reactor and heated at a temperature of 500°C for 2 hours in the absence of oxygen. The black residue was cooled and passed through a 0.5 mm sieve.


Characterization of biochars
The pH and electrical conductivity (EC) of the biochar were measured in a 1:20 (biochar: solution ratio) extract after shaking with deionized water for 1 h (Singh et al., 2010). The CEC was measured using 1 M ammonium acetate (pH 7) method (Lu, 1999). Ash content was determined using the ASTM D1762-84 method (ASTM International, 2013).Total C, H and N contents in the biochar were determined using an elemental analyzer (ECS 4010 CHNSO Analyzer). Specific surface area was analyzed by the Sear’s method (Sears, 1956). The surface morphology of adsorbents was characterized by Scanning Electron Microscope (SEM, AIS-2100, 5.0 kV, Korea).

Adsorption experiments
Batch experiments carried out with different initial concentration of Cd (0, 10, 20 , 40, 60, 80, 100, 150 and 200 mg L-1)at pH (4, 5, 6), ionic strengths (0.01, 0.03, 0.1 M) and temperature (10, 20, 30, 40 0C) at different times (0, 0.08, 0.17, 0.33, 0.67, 1, 2, 5, 10, 20, 40, 60, 90, 120 and 240 minutes) with 0.03 M NaNO3 as a background solution. Pseudo first-order, pseudo second-order, Elovich and fractional power were fit to experimental data to examine the adsorption kinetics of cadmium uptake by biochars. Non-linear fittness between experimental data and predicted values from adsorption isotherm (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich) was performed using the solver add-in with Microsoft’s spreadsheet, Microsoft Excel.
The amount of Cd2+ adsorbed per unit mass of the adsorbent (qad) was calculated by Eq. (1):

q_ads=C_i-C_e ×V/m (1)

where qads (mg/g) becomes qe or qt at equilibrium or at time t. Ci and Ce are the initial and the residual amount (mg) of Cd2+, respectively, added and remained in solution. V is the volume of the solution (mL), and m is the mass of adsorbents (g).
The removal efficiency (RE) was determined by computing the percentage sorption using the formulae in Eq. (2)

%RE=(C_i-C_e)/C_i ×100 (2)

Results and Discussion
Effect of Contact Time
The experimental data indicated that Cd adsorption increased with increasing contact time and the equilibrium time is attained in 40 min for grape and apple pruning residue biochar. Equilibrium time of 40 min was also reported by Rao et al. (2006) for Cd adsorption by activated carbon derived from Ceiba pentandra hulls.

Adsorption kinetics
Among adsorption kinetics models, pseudo second order model was better fitted for experimental data (R2=1) and the values of predicted equilibrium sorption capacities showed good agreement with the experimental equilibrium uptake values .Adsorption capacities (qe) obtained by pseudo second order model, were 9.8 and 9.0 mg g-1 for grape and apple pruning residue biochars respectively.

Adsorption isotherms
Ionic strength and pH of background solution significantly affected Cd adsorption and the highest adsorption capacity was obtained at pH 6, ionic strengths of 0.01M at 40 0C. Sorption capacity factors (qmax, KF, KT, qD) and sorption energy factors (n, KL, B) of gape pruning residue biochar was more than apple pruning residue biochar.
The sorption energy parameter (E) of Dubinin-Radushkevich isotherm and Gibbs free energy change (ΔG) indicated that the Cd adsorption processes were physical and spontaneous. The separation factor of Langmuir (RL) indicated that the sorption reactions of Cd2+ by grape and apple biochar is favorable (RL=0.15-0.50).

Conclusions
The results showed that, cadmium adsorption increased with increasing contact time and the equilibrium time is attained in 40 min for grape and apple pruning residue biochars respectively. The pseudo second order kinetic model accurately described the adsorption kinetics (R2=1). The highest adsorption capacity was obtained at pH 6, ionic strengths of 0.01M at 40 0C. It was observed that cadmium adsorption for grape pruning residue biochar was higher than apple pruning residue biochar. The separation factor of Langmuir (RL) indicated that the sorption reaction of Cd2+ biochars (RL=0.14-0.50) is favorable. Results from this study suggest that grape and apple pruning residue biochars were able to substantially remove Cd2+ from aqueous solutions.