Extracellular Polymers from Nitzschia sp. for Removing Clay Minerals from Water in Mining

Nitzschia sp., a diatom isolated from Paposo (Antofagasta, northern Chile), was evaluated as a biological solution for removing kaolinite-type clay minerals from recycled process water in large-scale copper mining. Optimization of culture conditions to maximize extracellular polymeric substance (EPS) production revealed that supplementing with 0.1 gL^-1 of glucose yielded the highest EPS levels on day 17, reaching 1285 ± 58.9 mgL^-1 (control equal to 237.8 ± 34 mgL^-1 on day 17). However, maximum dry weight biomass productivity was achieved in the presence of sodium carbonate at a concentration of 1 gL^-1 (319 ± 12.5 mgL^-1d^-1), significantly exceeding the productivity of the control group (242.7 ± 5.4 mgL^-1d^-1). Notably, low glucose supplementation enhanced EPS synthesis. Application of control-derived EPS of 1 gL^-1 rapidly decreased kaolinite initial turbidity from ~2024 FNU to ~354 ± 0.74 FNU within one minute. Even more glucose-derived EPS (1 gL^-1) further reduced turbidity to ~22.2 ± 0.1 FNU at 5 min, achieving a flocculation efficiency of ~98.9% after 15 min. Genomic analysis and KEGG annotation identified abundant genes for EPS and carbohydrate metabolism, including numerous glycosyltransferases, glycoside hydrolases, and multiple copies of UDP-glucose 4-epimerase, consistent with strong polysaccharide-biosynthesis capacity. Physicochemical characterization (particle sizing, HPLC, SEM, zeta-potential and FT-IR) showed EPS comprised mainly of rhamnose, fucose, arabinose, xylose and glucose, featuring functional groups (-OH, C=O/COO-, O-acetyl, uronic/guluronic signatures) that interact with kaolinite to promote aggregation. These findings demonstrate that Nitzschia-derived EPS, especially from glucose-supplemented cultures, represent promising sustainable bioflocculants for treating kaolinite-contaminated recycled water in mining operations.