Efficient Separation of Water-Soluble Humic Acid Using (3-Aminopropyl)triethoxysilane (APTES) for Carbon Resource Recovery from Wastewater

Humic substances are produced during the microbial degradation of biomolecules and should be recycled because they are an abundant source of carbon, hydrogen, oxygen, and nitrogen. Herein, (3-aminopropyl)triethoxysilane (APTES) was first used to recover carbon-rich water-soluble humic acid (HA) from wastewater, and the operational parameters (i.e., pH, solution temperature, APTES dosage, reaction time, and initial HA concentration) were optimized for separation efficiency. The results showed that the optimal HA separation efficiency (96.7%) could be achieved at an initial HA concentration of 250 mg/L, APTES dosage 4 mL/L, equilibration time 10 min, pH 3.0, and temperature 35 °C. Furthermore, cations (Mg²⁺ and Ca²⁺) led to a remarkable separation of HA, while anions (CO₃^2- and SO₄^2-) weakened the aggregation and deposition of HA. Mechanism analysis unveiled that ladderlike oligomeric aminosilanes (LAOAs) were generated by the hydrolysis, condensation, and self-assembly of APTES. The LAOAs were involved in adsorption bridging action with HA, thus efficiently promoting the precipitation of humic substances. The described carbon separation technique (converting dissolved matter into insoluble substances) can be a promising option for the efficient recovery and reutilization of carbon resources in high-concentration organic wastewater.