Multi-cycle regeneration of cation and anion exchange resins in a continuous-flow water treatment system

This study evaluates the regeneration performance and long-term operability of a continuous-flow, dual fixed-bed ion-exchange system for the simultaneous removal of lead (Pb²⁺) and nitrate (NO₃⁻) from water. Two acrylic columns (total height 45 cm; internal diameter 3.0 cm), each packed with 40 g of resin, were operated in series at pH 7.0 ± 0.1: a strong-acid cation exchanger (Purelite C100) for Pb²⁺ and a strong-base anion exchanger (Resinex™ NR-1) for NO₃⁻. Packed-bed heights were 8.0 cm (≈56.6 mL) for the cation column and 9.0 cm (≈63.6 mL) for the anion column. A 12-run Box–Behnken design investigated inlet concentration (40–80 mg L⁻¹), temperature (25–60 °C), and flow rate (40–100 mL min⁻¹) before and after regeneration with 10% (w/w) NaCl. Under optimized conditions (≈43 °C; 60 mL min⁻¹; 40 mg L⁻¹), Cycle 1 removals were 82.5% (Pb²⁺) and 92.3% (NO₃⁻). After six regeneration cycles, removals declined moderately to 70.2% and 83.6%, respectively, indicating good reusability with a slower efficiency decay for the anion resin. Quadratic response-surface models fit the data well (adjusted R² = 0.973 for Pb²⁺; 0.999 for NO₃⁻); concentration and flow were dominant negative factors, while elevated temperature mitigated mass-transfer limitations. A 10% NaCl protocol is therefore an effective baseline for routine regeneration, with scope for further capacity retention via longer brine contact, occasional deep-clean steps, or tailored regenerant dosing.