Abstract

Acid mine drainages (AMD) are the most harmful and chemically complex aqueous waste produced by the mining industry. Their high heavy metals and sulfate contents cause contamination of fresh water sources, affecting human communities and environment. Complexity of AMD makes it difficult to treat using traditional methods, which has led to the search for sustainable approaches capable of reusing and recovering water. In this context, membrane distillation (MD) is an alternative technology based on transmembrane water partial pressure as a driving force to transport vapor water molecules across a hydrophobic membrane. However, MD has some disadvantages, such as low productivity, membrane fouling, and wetting, which causes a pressing need to use new materials with improved properties. This research focuses on novel hydrophobic membranes (PVDF modified by grafting incorporation of 1H,1H,2H,2H-perfluorodecyl-triethoxysilane (PDTS)), which mitigate the wettability and potentially improve efficiency of MD process. Membranes were characterized by Scanning Electron Microscopy/Energy Dispersive X-Ray (SEM/EDX), Fourier Transform Infrared (FTIR) spectroscopy and water contact angle (WCA) and evaluated in a Direct Contact Membrane Distillation (DCMD) laboratory-scale setup (1L) using synthetic AMD solution consisting of copper sulfate (CuSO4) with concentrations similar to real AMD. A Box-Behnken experimental design (BBD) was applied to optimize three MD operating parameters: crossflow velocity (ν), temperature (T°), and feed solution concentration (AMD_c). Results obtained from BBD were analyzed by Analysis of variance (ANOVA) and Response Surface Methodology (RSM) to determine impact of evaluated parameters together with optimal operating conditions to maximize permeate flux (F_p). Finally, these optimal conditions were validated for both synthetic and real AMD solution (obtained from Yerba Loca, Metropolitan Region, Chile) with the membranes. Characterization results confirm the incorporation of PDTS on the material with good dispersion on the surface, without affecting the chemical composition and improving membrane hydrophobicity, increasing WCA by 12% (114°±1.92 for pristine membrane vs 139°±0.48 for modified membrane). BBD experimental results showed improvements in the F_p of the modified membranes compared to the unmodified under all evaluated operating conditions. Statistical analysis showed that permeate flux is strongly influenced by of T° and ν. In addition, results obtained under optimal operating conditions in DCMD process improved the F_p by 23% and the water recovery (WR) by 43% in modified membranes in high AMD concentrations (2500 ppm), obtaining great concordance with predicted value obtained by RSM (Table 1). These results were also observed using real AMD solution as feed, with an improvement of 20% in F_p and 14 % in WR. PDTS incorporation in PVDF membrane was validated by SEM and FTIR characterizations, while WCA showed increased membrane hydrophobicity. The BBD experimental results demonstrate an improvement in both F_p and WR compared to pristine membrane in all AMD concentrations tested, including real AMD samples. Thus, this study provides a significant and innovative contribution to sustainable treatment of AMD solutions, presenting answers for water resource management in the mining industry.