Optimization preparation of biochar from garden waste and quantitative analysis for Cd²⁺ adsorption mechanism in aqueous solution

To develop a market for biochar, it is imperative that solutions are found to producing biochars that are both high performance and economically viable. While biochar performance can be enhanced via chemical modification, it is likely that optimization of pyrolysis time and temperature is a more cost-effective approach to enhancing performance. This was explored via the transformation of urban garden waste into biochar using a range of preparation conditions (heating temperature, residence time, and heating rate). Biochar yield and Cd²⁺ adsorption performance were optimized using response surface methodology. The “best compromise” yield and Cd²⁺ adsorption performance (49.9% and 40.0 mg/g, respectively) of garden waste biochar were achieved using preparation conditions of 398 ℃, 10 ℃/min, and 30 min. In addition, the quantification of adsorption mechanisms suggested mineral precipitation, ion exchange, functional group complexation, and physical adsorption, accounted for 47.9%, 41.5%, 10.3%, and 0.3% of total adsorbed Cd²⁺ in biochar, respectively. Overall, transformation of garden waste into adsorbents might offer a new market for the utilization of urban garden waste, especially given the size of this waste stream and the challenges it presents to municipal administrations. Graphical abstract: (Figure presented.)