Low cost pineapple peel biosorbent for the removal of aqueous cobalt from textile wastewater effluent

Abstract

The concept of waste and waste management is a Civil Engineering problem. Billions of Rands is spent each year to cater for industrial and municipal waste of a constantly increasing population. Traditional waste management practice prioritises landfilling as a primary and ultimate means of waste disposal. All capital-intensive projects end up transferring the original waste from one form to another or entombing the waste as is the case with landfilling. An alternative solution to the conventional practices in sustainable waste management approach, embraces the integration of renewable resources such as biomass use as an economically viable option. The advantage of such an integrated approach includes the elimination of waste transportation, landfilling, and ultimately the reduction of the ecological footprint. In this study, a sustainable waste management strategy was explored by converting waste material into a potentially valuable product. Pineapple peels were applied for the biosorptive removal of cobalt (a class B carcinogen) from aqueous solution. Pineapple peels were collected from a communal residential villa and local vegetable retail outlet. Samples were dried, ground, sieved (150 µm) and subjected to chemical (KMnO4 +H2SO4) and physical (carbonisation at 600°C in Nitrogen) treatments. The effect of physicochemical treatment and carbonisation on biosorption was thus investigated. In all, one pristine and two modified samples were obtained; raw pineapple peels (RPP), chemically treated pineapple peels (CTPP) and carbonised pineapple peels (CPP). Activated carbon (AC) served as an experimental control. The bio-sorbents were characterized by Fourier transform infrared spectroscopy (FTIR), Brunauer Emmet Teller (BET) and scanning electron microscopy (SEM) to identify the functional groups in biosorbent and assess changes to surface area and morphology. Biosorption experiments were conducted in batch and continuous mode with the residual cobalt determined by atomic absorption spectroscopy (AAS). For batch experiments, the optimum operating variables, biosorbent mass and initial pH, were found to be 0.05 g and pH 6, respectively. Maximum biosorption capacities of 8.958 mg/g, 16.731 mg/g, 22.314 mg/g and 6.337 mg/g were obtained for RPP, CTPP, CPP and AC, respectively. Carbonised samples displayed exothermic character while biosorption on non-carbonised samples was endothermic. Biosorption was rapid, reaching equilibrium in 60 minutes. Batch kinetic data were in agreement with the pseudo-second-order model, while equilibrium data was better described by the Freundlich isotherm models (R² ≈ 0.99) on the CPP and CTPP samples. Column studies using CPP expectedly showed that breakpoint and saturation point was directly related to column mass and inversely related to solution flow rate and feed concentration. The maximum saturation capacity for CPP in column mode was 16.009 mg/g, achieved when the concentration of cobalt feed, flow rate, and the adsorbent bed was 25 ppm, 5 mL/min and 1 g, respectively. In general, column data were best fitted to the Yoon model, when compared to the Thomas and Bohart Adams models. Predicted maximum adsorption capacity by the Bohart Adams model did not agree with experimental data. Based on the data obtained, the biosorbent was successfully applied to treat industrial textile wastewater. In conclusion, it was found that the biosorbent made from pineapple peels were suitable, especially when subjected to physical treatment for the biosorptive removal of cobalt ions from aqueous solution and gave better results in comparison to activated carbon used as control. As a novel addition to the existing body of literature, it was noted that the issue of residual colour observed when pineapple peels were used in their pristine form could be overcome by using carbonised pineapple peels. These peels which are vegetable wastes from retail shops is hereby proposed as a suitable candidate for a green approach to water treatment for cobalt contaminated water.