Natural Rubber Sheet Production Wastewater Properties and Treatability by Zero-Valent Iron-Modified Agricultural Residual Biochar for Sustainable Treatment

The production of one ton of natural rubber sheets generates 10–18 m3 of natural rubber sheet production wastewater (NRW), containing diammonium phosphate, tetramethylthiuram disulfide/zinc oxide (TMTD/ZnO), and formic acid. This study evaluated palm shell biochar (PSB), Longkong wood biochar (LWB), and their zero-valent iron (Fe0)-modified forms (Fe0-PSB and Fe0-LWB) for NRW remediation at dosages of 0.5, 1, 2, 5, 10, and 20 g/L over 168 h. At 0.5 g/L, PSB exhibited maximum adsorption capacities of 3.631, 1.084, 3.670, and 2.679 mg/g for ammonium, nitrate, phosphate, and TMTD, respectively, whereas LWB required higher dosages (2 and 5 g/L) to achieve comparable adsorption capacities. Fe0 modification significantly enhanced the removal performance of both biochars. Fe0-LWB showed superior adsorption capacities for nitrate (1.301 mg/g) and phosphate (48.052 mg/g), while Fe0-PSB performed better for ammonium (15.072 mg/g) and TMTD (4.686 mg/g). Compared with biochars, Fe0-LWB and Fe0-PSB increased ammonium removal from <5% to 14.61% and 40.75%, and phosphate removal from <1% to 20.99% and 24.01%, respectively. This enhancement is driven by various iron species via redox reactions, modulated surface charge, enhanced electrostatic attraction, and surface precipitation. Therefore, Fe0-modified biochars offer a sustainable NRW treatment strategy by valorizing biomass-derived materials as effective adsorbents.