Bacteria-polysaccharide interfacial regulation by calcium alginate hydrogel capsules supports sulfate reduction in acidic water.

Calcium alginate hydrogels are widely used for microbial encapsulation, yet their role in regulating bacteria-polysaccharide interfaces under acidic sulfate conditions remains unclear. Here, calcium alginate hydrogel capsules (SAC) were used to encapsulate a functional sulfate-reducing consortium and evaluate its performance in acidic sulfate-containing water. At pH 4, SAC maintained live-cell densities 147-185 times higher than those of free cells and achieved higher sulfate removal rates on days 7 and 14 (19.9% and 21.3%) than free cells (3.7% and 10.0%). Inactivated SAC showed limited sulfate removal (5.5%-5.9%), indicating a substantial live-cell-associated contribution. The Ca-alginate network retained a porous structure and exhibited enhanced rheological resistance, with shear stress increasing from 152.98 to 548.1 Pa after 7 days of acid stress. Extracellular polymeric substances (EPS) analysis indicated compositional remodeling rather than increased accumulation, while electrochemical OCP relaxation showed a reduced apparent decay time constant from 1.167 s in free cells to 0.508 s in SAC, suggesting faster interfacial chemical-state recovery. These findings demonstrate that Ca-alginate capsules enhance acid tolerance through cell retention, matrix stability, EPS-interface remodeling, and altered proton-activity relaxation, offering a polysaccharide-based strategy for sustaining microbial function in acidic sulfate-containing wastewater.