Health risk assessment and removal of nitrate and sulfate ions using Mg/Al quartz-based geopolymer: a case study from Siwa Oasis

This study assesses the health risks associated with nitrate and sulfate contamination in surface and groundwater sources within the Siwa Oasis, located in Egypt's northwestern desert. It also explores the effectiveness of a low-cost magnesium/aluminum quartz-based geopolymer (Mg/GP) as an enhanced adsorbent for contaminant removal. Nitrate concentrations reached up to 29.4 mg/L, exceeding acceptable safety limits, as indicated by a total hazard quotient (THQ) greater than 1. This level of exposure presents significant health risks, particularly for children (oral HQ up to 1.23). Sulfate concentrations also surpassed irrigation standards, further highlighting concerns regarding water quality in the region. To mitigate these risks, Mg/GP was synthesized as a modified, cost-effective geopolymer exhibiting high adsorption capacities for both nitrate (166.1 mg/g) and sulfate (234.1 mg/g) ions. The adsorption behavior followed pseudo-first-order kinetics (R² > 0.91) and was well-described by the Langmuir isotherm model (R² > 0.99). Additionally, statistical physics-based equilibrium modeling was employed to analyze steric and energetic adsorption characteristics. The analysis of active site density revealed a higher saturation capacity for sulfate (Nm = 145.4 mg/g) compared to nitrate (Nm = 18.8 mg/g), indicating more efficient sulfate uptake. However, nitrate ions demonstrated a stronger aggregation affinity (n = 8.83) than sulfate (n = 1.6), with both values exceeding 1, suggesting the presence of multi-ionic interaction mechanisms. These interactions were predominantly physical and exothermic, as evidenced by adsorption energies (ΔE < 20 kJ/mol). Field application of Mg/GP for groundwater treatment confirmed its practical potential, achieving a 72.4% reduction in sulfate concentration (initially 456.2 mg/L) and a 94.2% reduction in nitrate concentration (initially 10.4 mg/L).