Optimization of boron doped eucalyptus biochar for removal of phenolic compounds from municipal wastewater

Abstract

The discharge of phenolic compounds into water bodies poses significant environmental and public health risks due to their toxicity, persistence, and resistance to natural degradation. This study focused on the synthesis, characterization, and evaluation of boron-doped eucalyptus biochar as a low-cost adsorbent for phenol removal from aqueous solutions and municipal wastewater. Eucalyptus biomass was collected from local plantations in Bindura, subjected to pyrolysis at 500°C for 2 hours under limited oxygen conditions, yielding approximately 32.4% biochar. Boron doping was achieved through chemical impregnation using boric acid and borax at varying concentrations (6.25% to 25.00%), followed by thermal activation at 700°C. Fourier Transform Infrared Spectroscopy confirmed successful boron incorporation through the appearance of B-O stretching vibrations. Batch adsorption experiments were conducted to evaluate the effects of boron concentration, adsorbent dosage, solution pH, contact time, and reusability on phenol removal efficiency. The optimal boron concentration was found to be 16.66%, achieving 91.3% phenol removal. Maximum removal efficiency of 93% was attained at an adsorbent dosage of 0.8 g per 100 mL. Solution pH significantly influenced adsorption, with optimal performance observed at pH 7, which corresponded to the point of zero charge determined through the pH drift method (pHpzc = 7.0). Contact time studies revealed rapid adsorption within the first 60 minutes (78% removal), reaching equilibrium at 120 minutes. Kinetic analysis demonstrated that the adsorption process followed the pseudo-second-order model (R² = 0.9963), indicating chemisorption as the rate-limiting step. The Langmuir isotherm model best described the equilibrium data, suggesting monolayer adsorption onto homogeneous surface sites. Reusability studies showed that the biochar retained 78.6% removal efficiency after five consecutive cycles. Application of the optimized biochar to municipal wastewater from the Astra Treatment Plant in Bindura resulted in significant reductions: phenol (55.6%), turbidity (85.7%), conductivity (40.4%), biochemical oxygen demand (63.2%), and chemical oxygen demand (61.0%). The study concludes that boron-doped eucalyptus biochar is an effective, sustainable, and cost-efficient adsorbent for phenol removal, offering a viable wastewater treatment solution particularly suitable for resource-limited settings. Recommendations include pilot-scale investigations, advanced characterization studies, and economic feasibility assessments to support large-scale implementation.