Nitrate reduction by mixed iron(II-III) hydroxycarbonate green rust in the presence of phosphate anions: The key parameters influencing the ammonium selectivity

• Published in: Water research (Oxford) 2014-10, Vol.62, p.29-39
• Main Authors: Etique, Marjorie, Zegeye, Asfaw, Grégoire, Brian, Carteret, Cédric, Ruby, Christian
• Format: Article
• Language: English
• Subjects: Ammonium Compounds - chemistry; Anions; Applied sciences; Biological and physicochemical phenomena; Carbonates - chemistry; Continental interfaces, environment; Denitrification; Environmental Sciences; Exact sciences and technology; General purification processes; Green rust; Iron; Iron - chemistry; Kinetics; Layered double hydroxide; Natural water pollution; Nitrates; Nitrates - isolation & purification; Oxidation-Reduction; Passive treatment; Phosphates; Phosphates - chemistry; Pollution; Reduction; Saturation; Sciences of the Universe; Selectivity; Wastewaters; Water treatment and pollution
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2014.05.028

The reduction of nitrate anions by a mixed FeII–FeIII carbonated green rust (GR) in aqueous medium is studied as a function of the initial pH and the initial concentrations of iron, phosphate and nitrate. The influence of these parameters on the fraction of nitrate removed and the production of ammonium is investigated by the help of statistical experimental designs. The goal is to determine experimental conditions that maximize the fraction of NO3− removed and concomitantly minimize the production of NH4+. Increasing the phosphate concentration relatively to the initial FeII concentration inhibits the reduction of nitrate probably due to a surface saturation of the lateral sites of the GR crystals. The kinetics of the reaction is greatly enhanced by increasing the initial pH at 10.5, however it leads to a global increase of the NH4+ production. A partial saturation of the surface sites by phosphate leads to a global decrease of selectivity of the reaction towards ammonium. The evolution of the ratio of the NH4+ concentration to the FeII concentration confirms that the NO3− species are only partially transformed into ammonium. Interestingly at an initial pH of 7.5, the selectivity of the reaction towards NH4+ is often lower than ∼30%. The reduction of nitrate by carbonated GR differs from the behavior of other GRs incorporating Cl−, F− and SO42− anions that fully transform nitrate into ammonium. Finally, if GR is intended to be used during a passive water denitrification process, complementary dephosphatation and ammonium treatments should be considered. The selective reduction of nitrate in water is achieved by using green rust in the presence of a moderate amount of phosphate. This study can be used as a guide for designing future experiments in hydrodynamic conditions where complementary dephosphatation and/or elimination of residual ammonium should be considered. [Display omitted] •Reactivity of green rust with NO3− is studied with a full factorial design.•Parameters are pH and nitrate, iron(II) and phosphate concentrations.•Responses are the nitrate reduction and ammonium production ratios.•NH4+ production ratio can be as low as 5–30%.•Water dephosphatation should be performed before water denitrification.