Trimethylsilane-Grafted Low-Rank Coal-Based Activated Coke for Hydrophobic Adsorption of Indole and Diethyl Phthalate from Wastewater

Poorly soluble hydrophobic organic pollutants, such as indole and diethyl phthalate (DEP), are difficult to remove efficiently from complex industrial wastewater due to low solubility and competitive adsorption. In this study, low-rank coal-based activated cokes derived from Wanli long-flame coal and Zhaotong lignite were modified through a combined process of acid-washing pretreatment and trimethylchlorosilane (TMCS) grafting. The acid-washing step effectively removed ash and unblocked pores, increasing the specific surface area and pore volume of the optimized Zhaotong lignite-based sample by 43.7% and 53.3%, respectively. Subsequent TMCS grafting successfully introduced hydrophobic methyl groups onto the surface, significantly enhancing hydrophobicity. The water contact angles of the composite materials (acid-washed plus TMCS-grafted) increased to 127.3° and 139.7°, compared to 117.8° and 112.6° for the original samples. The modified adsorbent derived from Zhaotong lignite exhibited high adsorption capacities, reaching 139.47 mg·g−1 for indole and 120.19 mg·g−1 for DEP in single-component systems, representing an increase of 20.1% for indole and 28.7% for DEP compared to the unmodified adsorbent. More importantly, in a competitive system containing phenol at PH = 10, the materials demonstrated superior selectivity towards the target hydrophobic pollutants. The phenol removal rate was 65.97%, and the removal rates for indole and DEP increased sharply to 98.17% and 92.17%, respectively. This work provides a feasible strategy for the advanced treatment of complex organic wastewater using coal-based adsorbents, achieving a dual enhancement in both adsorption capacity and selectivity.