Removing nitrate from water using iron-modified Dowex 21K XLT ion exchange resin: Batch and fluidised-bed adsorption studies

• Fe modification of anion exchange resin increased nitrate adsorption.
• Fluidised-bed adsorption data well fitted to Thomas and plug-flow models.
• EDS and zeta potential data explained the adsorption mechanism.
• Nitrate adsorption reduced greatly by sulphate but little by phosphate and chloride.

Elevated concentrations of nitrate in surface waters can cause eutrophication, while in drinking water they pose a threat to human health, especially causing blue baby syndrome in infants. An anion exchange resin – Dowex 21K XLT – was surface modified by incorporating Fe (Dowex-Fe) and tested to remove nitrate from aqueous solutions in batch and fluidised-bed adsorption experiments. Solution pH in the 4.0–7.5 range had no effect on nitrate adsorption. The adsorption data at pH 6.5 fitted well to the Langmuir model with maximum adsorption capacities of 27.6 mg N/g, and 75.3 mg N/g for Dowex and Dowex-Fe resins, respectively. Energy dispersion spectrometry and zeta potential measurements indicated that the increase in adsorption due to Fe impregnation was caused by additional surface positive charges induced on the resin by Fe. In both batch and fluidised-bed experiments, nitrate adsorption capacity declined markedly when sulphate was added, but phosphate and chloride additions had little effect. Batch kinetic data fitted well to the pseudo-first, pseudo-second and homogeneous surface diffusion models. Data from the fluidised-bed experiments satisfactorily fitted to the Thomas and plug-flow models. More than 95% of adsorbed nitrate was desorbed by 1 M KCl in all three adsorption/desorption cycles and the resins were successfully regenerated in each cycle with little reduction in adsorption capacity. No significant reduction in the Fe content of Dowex-Fe occurred during the regeneration.