Adsorption characteristics of lead (Pb) from aqueous solutions by grape pruning residues and their biochar

Lead (Pb) is a toxic heavy metal and is a ubiquitous contaminant in terrestrial and aquatic environments. The major inputs of Pb in aquatic systems come from drainage and surface runoffs effluent discharges from industries. Lead could be removed effectively by adsorption on a range of natural adsorbents. Biochar is a relatively novel sorbent produced by the pyrolysis of a feedstock under oxygen-limited or anaerobic conditions and usually has adsorption ability for heavy metals due to its higher surface area and cation exchange capacity. Agricultural residues, especially grape pruning residues, being produced in large quantities in the vineyards, are inexpensive and efficient biosorbents for Pb removal, hence, this study aimed to examine the potential mechanisms of Pb removal from aqueous solution by grape pruning residues and its biochars.

In order to study the lead (Pb) adsorption behavior by grape pruning residue and its biochar, batch experiments carried out with different initial concentration of Pb (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) were investigated.

The results showed that the adsorption capacity of adsorbents increased with increasing initial concentration, pH and decreasing ionic strengths. The adsorption data were well fitted with Langmuir and Dubinin-Radushkevich models compared to Freundlich and Temkin models. Sorption capacity factors (qmax, KF, B, qD) and sorption energy factors (n, KL, KT) of gape pruning residue biochar was more than grap pruning residue. Temperature of background solution significantly affected Pb adsorption and the highest adsorption capacity was obtained at 40 0C. The sorption energy parameter (E) of Dubinin-Radushkevich isotherm (3.6 to 7.6 kJ mol-1) and negative Gibbs free energy (∆G) values (-16 to -21 kJ mol-1) revealed the physical adsorption and spontaneous of Pb adsorption on the grape pruning residue and its biochar, respectively. The entropy (ΔS) and change in enthalpy (ΔH) were found to be 0.002 J mol-1 K-1 and 0.31 kJ mol-1 for grape pruning residue and 0.002 J mol-1 K-1 and 0.40 kJ mol-1 for grape pruning residue biochar, reflecting an affinity of Pb on the bioadsorbents and endothermic nature of Pb adsorption reaction.

This study revealed that the optimized pH and ionic strengths to reach the maximum sorption could be obtained based on the isotherm experiments, while the thermodynamic investigations could be of help to find the optimum temperature to achieve the most effective sorption by given adsorbent. Results from this study suggested that grape pruning residue and its biochar are effective adsorbent for the removal of Pb from wastewater, since it is a low-cost, abundant and locally available.