A macromolecular oxidant, the N,N-dichlorosulfonamide for removal of residual nitrites from aqueous media

Nitrites are highly harmful compounds. They are extremely undesirable in surface and municipal water. The permissible content in natural water is very low and should not exceed 0.01 mg of NO2-/dm3. In this instance a redox copolymer, a macromolecular analogue of Dichloramine T (i.e. a macroporous S/DVB copolymer containing -SO2NCl2 groups) was used for the removal of nitrite ions from aqueous solutions by its oxidation to achieve the less toxic nitrates. The resin was prepared starting from Amberlyst 15 by a three-step transformation of the sulfonic - via chlorosulfonyl and sulfonamide - to the N,N-dichlorosulfonamide groups. The resulting copolymer contained 8.2 meq of active chlorine/g and showed strong oxidizing properties. It was employed in batch and flow processes for treatment of NaNO2 solutions containing 115 and 230 or 460 mg of NO2-/dm3. The effects of various parameters on the reaction course have been studied (mole ratio of reagents, pH of the reaction media, flow rate in the column processes). The solid phase oxidation carried out in a dynamic regime provided the drive to bring the reaction to completion. Thus, nitrite free effluents (<3.0 μg/dm3) were obtained in the column processes. The reaction of nitrite oxidation by means of this heterogeneous oxidant was fast, and therefore the permissible flow rate was highly satisfactory - close to 20-25 bed volumes/h. Under the examined reaction conditions, 1 mol of NO2- ions was oxidized by 1 mol of active chlorine, thus the oxidation capacity of the resin was nearly 200 mg NO2-/g of the copolymer. The N,N-dichlorosulfonamide copolymer is very useful for the purification of neutral solutions from nitrites. The copolymers redox potentials determined here and the complex redox titration measurements which have been carried out, prove why the macromolecular oxidant shows varied reactivity, depending on the solution's pH.