Mechanisms and optimization for simultaneous removal of Cd(II) and Sb(V) from aqueous solutions using birnessite and fulvic acid composite.

Cadmium (Cd) and antimony (Sb) coexistence in industrial effluents poses significant threats to environmental safety and human health. Consequently, developing effective methods for the simultaneous removal of Cd(II) and Sb(V) from aqueous solutions is critically important. In this study, the adsorption performance of a birnessite (BS) and fulvic acid (FA) composite (BS-FA) for the simultaneous removal of Cd(II) and Sb(V) was optimized using response surface methodology (RSM) in combination with machine learning (ML) techniques, including the genetic algorithm-back propagation neural network (GABP) and random forest (RF) models. The RF model demonstrated superior predictive accuracy (R² = 0.8037, RMSE = 0.0625) compared to the RSM and GABP models. Under the optimized conditions (pH = 6, adsorbent dosage = 0.87 g L, adsorption time = 4 h, ionic strength = 0.01 mol L⁻¹, initial concentration = 25.5 mg L⁻¹), the removal efficiencies of Cd(II) and Sb(V) were 96.9% and 70.2%, respectively. Microscopic and mechanistic analyses revealed that Cd(II) and Sb(V) interacted with the Mn-O bonds in BS and the oxygen-containing functional groups (C-OH and -COOH) in FA, forming stable complexes within the Cd-Sb coexistence system. This study successfully integrates ML models and RSM to optimize and predict the adsorption process, offering valuable insights for mitigating the environmental and health risks associated with Cd and Sb contamination in water treatment.