Biochar and kinetics studies on the reduction of sodium bromate by a cobaloxime in an aqueous media: How we can remove a toxic substance from our environment

Abstract

The reduction of sodium bromate (NaBrO₃) by [Co(dmgBF₂)₂(OH₂)₂] (where dmgBF₂ = difluoroboryldimethylglyoximato) was investigated in hydrochloric and nitric acids. Analytical studies were carried out via gravimetric analysis and UV–visible spectrophotometry. Gravimetric analysis proved that sodium bromate was reduced to from quantitative amounts of sodium bromide in the presence of hydrochloric acid. Biochar was used to sequester the complex from the reaction mixture and to neutralize the acidic media. The UV–visible data confirmed the sequestration of the complex from the reaction mixture after exposure to the biochar. The collected pH data proved a direct relationship between pH and the amount of biochar used in the form of a titration curve. Kinetic studies were also carried out to ascertain the mechanism of the oxidation of [Co(dmgBF₂)₂(OH₂)₂] by NaBrO₃. The oxidation of [Co(dmgBF₂)₂(OH₂)₂] by NaBrO₃ was carried out by stopped-flow spectrophotometry at 450 nm by varying temperature and over the range of 1.00 mM ≤ [HCl] ≤ 11.00 mM, at a constant ionic strength of 0.60 M (NaCl). From the data, a mechanism for the reaction was proposed. From the mechanism the following rate expression was derived, ${\text{k}}_{\text{obs}}=\frac{\left({\text{k}}_2{\text{K}}_{\text{a}}\text{;}+\text{;k}{\text{;}}_1\left({\text{H}}^{+}\right)\right){\left({\text{BrO}}_3^{-}\right)}_{\text{T}}}{\left[{\text{H}}^{+}\right]\text{;}+\text{;}{\text{K}}_{\text{a}}}$, where k₁ was calculated to be 4.2 × 10⁴ M⁻¹ s⁻¹ at 25 °C, and the activation parameters (ΔH^‡ and ΔS^‡) were calculated as 57 ± 1 kJ mol⁻¹ and 34 ± 4 J mol⁻¹ K⁻¹, respectively.