Efficient Removal of Ammonium, Heavy Metals, and Scale-Forming Cations from Oilfield Produced Water by Sulfonated Biochar

The treatment of oilfield produced water (OPW), which contains complex pollutants, such as scale-forming cations, heavy metals, ammonia, and organic matter, presents a significant challenge. In this study, we utilized low-cost sulfonated biochar (SBC), produced through in situ sulfonation of alkylated waste sulfuric acid with waste rice husk, to remove these pollutants from actual OPW. Treatment with 60 g/L of SBC achieved removal rates of 65.7% for COD, 66.1% for DOC, 89.6% for NH4+–N, and over 96.3% for scale-forming cations (Ba2+, Ca2+, and Mg2+), with heavy metals (Cd2+ and Cu2+) being completely removed (100%). SBC effectively adsorbed cationic pollutants via surface functional groups (−SO3H, −OH, and −COOH) and removed diverse organic pollutants (highly unsaturated compounds, polyphenols, and polycyclic aromatics) through pore-filling, electrostatic interactions, and π–π interaction mechanisms. SBC released substantial quantities of sulfur- and oxygen-containing compounds, inducing a pH decrease in OPW from 6.97 to 3.05. Approximately 30% of these compounds are microbially bioactive molecules (H/C ≥ 1.5). Moreover, SBC proved effective for OPW treatment, maintaining a stable adsorption performance through six consecutive cycles. These findings suggest that SBC holds great potential for large-scale applications as an adsorbent in the pretreatment stages of OPW treatment processes.