Surface activation of rice husk-derived hydrochar as an adsorbent for atrazine and ammonium

Abstract

Due to the agricultural intensity, studies of sustainable solutions for atrazine and nitrogen contamination have attracted more attention. This study is aimed to develop rice husk hydrochar as an adsorbent for atrazine and ammonium adsorption. Effects of microwave-assisted hydrothermal carbonization (MHTC) conditions including temperature (150 – 200°C), residence time (20 – 60 min), and liquid to solid ratio (5:1 – 15:1 mL/g) on the atrazine and ammonium adsorption capacity were investigated. Surface activation of rice husk hydrochar samples was synthesized using potassium hydroxide (KOH) and hydrogen peroxide (H2O2) with various concentrations. The results showed that higher MHTC temperature with higher residence time and lower liquid to solid ratio significantly increased the adsorption capacity of these contaminants on rice husk hydrochar. The characterization of pristine and activated hydrochars using BET, FT-IR, and XPS revealed that rice husk hydrochar contained abundant oxygen-containing functional groups, which increased even more after KOH treatment. The results of kinetic and isotherm studies suggested that atrazine adsorption was best fitted with pseudo-second-order and Freundlich models (R2 > 0.99), with the maximum adsorption capacity of 4.06 mg/g for KOH-activated hydrochar. Hydrogen-bonding and π-π electron donor-acceptor interactions played dominant roles for atrazine adsorption of KOH-activated hydrochar and H2O2-activated hydrochar, respectively. Compared to atrazine, the ammonium adsorption onto rice husk hydrochar was very low even though the KOH treatment enhanced its CEC values. Therefore, rice husk hydrochar can be considered a promising candidate for atrazine adsorption in the environment, and future studies are recommended to apply this material as a nitrogen slow-release fertilizer.