Template-free Synthesis of Zn−P-Doped Carbon Microspheres from Corn Distiller Solubles via Surface-Engineered Hydrothermal Processing: A Promising Cationic Dye Adsorbent

Abstract The rising global demand for sustainable resources has accelerated interest in biomass-derived carbons for environmental remediation. However, the valorization of agro-industrial byproducts into high-value functional materials remains a key challenge in advancing circular bioeconomy pathways. This study presents a sustainable template-free strategy for synthesizing zinc phosphate hybrid carbon microspheres (Zn−P/C) from corn distiller’s solubles (CDS), an underutilized agro-industrial byproduct for methylene blue adsorption application. A two-step process, consisting of mild H3PO4 pretreatment followed by ZnCl2-assisted hydrothermal carbonization (HTC), was developed to introduce phosphate functionalities and enable in situ formation of crystalline zinc phosphate within a carbon matrix. Pretreatment conditions, including H3PO4 concentrations of 1.0−2.0 M, temperatures of 25−75 °C, and durations of 60−180 min, were optimized to enhance substrate reactivity, reduce ash content, and incorporate phosphate groups. The pretreated feedstock underwent carbonization during HTC at 180−220 °C, forming porous carbon microspheres with embedded crystalline zinc phosphate. The resulting hybrid materials (Zn−P/C) achieved an enhanced carbon content of 74.07 wt %, structure stability, improved H/C and O/C ratios, abundant functional groups, and uniform microspherical morphology as confirmed by FTIR, XRD, and SEM analyses. The resulting hybrid adsorbent Zn7-P/C-220 achieved a superior dye removal efficiency of 98.2%, outperforming many conventional biocarbon based adsorbents. The strategy in this study offers a scalable, low-temperature route to convert CDS into an engineered multifunctional high-performance hybrid carbon with potential applications in advanced adsorption systems, catalysis, and energy storage while contributing to sustainable resource utilization and industrial waste valorization.