Synthesis and assessment of phosphoric acid-activated biochar from watermelon (citrullus lanatus) peels for the removal of Lead (II) and Cadmium (II) ions from aqueous media

Abstract

This study investigated a sustainable, circular-economy approach to address the dual environmental challenges of heavy metal water pollution and agricultural waste management in Kampala, Uganda. Activated biochar was synthesized from locally sourced Citrullus lanatus (watermelon) peels via chemical activation with phosphoric acid (H3PO4) followed by pyrolysis. The activation process was optimized by varying the pyrolysis temperature (400 °C, 500 °C, and 600 °C), with 500 °C identified as the optimal temperature, balancing biochar yield (38.1%) and the development of favorable adsorbent properties. The resulting watermelon peel activated biochar (WP-AC) exhibited an acidic surface with a point of zero charge (pH pzc) of 3.8. Batch adsorption experiments were conducted to evaluate the efficacy of WP-AC for removing lead (Pb (II)) and cadmium (Cd (II)) from synthetic aqueous solutions. The adsorption process was highly pH-dependent, with optimal removal achieved at a solution pH of 5.5. Kinetic studies revealed that the adsorption process for both metals was best described by the pseudo-second-order model (R2 > 0.99), indicating that chemisorption is the rate-limiting step. Equilibrium data were wellfitted by the Langmuir isotherm model (R2 > 0.99), suggesting monolayer adsorption onto a homogeneous surface. The WP-AC demonstrated a high adsorption capacity, with maximum monolayer adsorption capacities (q max) determined to be 118.5 mg/g for Pb (II) and 85.2 mg/g for Cd (II). The superior adsorption of Pb (II) was attributed to its higher electronegativity and lower hydration enthalpy compared to Cd (II). The primary removal mechanisms were identified as combining of electrostatic attraction, ion exchange, surface complexation with oxygen- and phosphorus-containing functional groups, and surface precipitation. These findings highlight that activated biochar derived from watermelon peels is a highly effective, low-cost adsorbent, presenting a viable and locally relevant solution for remediating heavy metal-contaminated water in regions like Kampala.