Promising Biocompatible, Biodegradable, and Inert Polymers for Purification of Wastewater by Simultaneous Removal of Carcinogenic Cr(VI) and Present Toxic Heavy Metal Cations: Reduction of Chromium(VI) by Poly(ethylene glycol) in Aqueous Perchlorate Solutions

Abstract

A spectrophotometric technique has been applied for studying the reduction of chromium(VI) by poly(ethylene glycol) (PEG) as water-soluble and nontoxic synthetic polymer at a constant ionic strength of 4.0 mol dm^-3 in the absence and presence of the ruthenium(III) catalyst. In the absence of the catalyst, the reaction orders in [Cr(VI)] and [PEG] were found to be unity and fractional first orders, respectively. The oxidation process was found to be acid-catalyzed with fractional second order in [H⁺]. The addition of Ru(III) was found to catalyze the oxidation rates with observation of zero-order reaction in [CrO₄ ^2-] and fractional orders in both [PEG] and [Ru(III)], respectively. The PEG reduces the soluble toxic hexavalent Cr(VI) as a model pollutant to the insoluble nontoxic Cr(III) complex, which is known to be eco-friendly and more safer from the environmental points of view. The acid derivative of PEG was found to possess high affinity for the removal of poisonous heavy metal ions from contaminant matters by chelation. Formation of the 1:1 intermediate complex has been kinetically revealed. A consistent reaction mechanism of oxidation was postulated and discussed.

Figure 1

FTIR Spectra of PEG and its acid derivative of PEG.

Figure 2

Plot of 1/k_obs vs 1/ [PEG]. [CrO₄^2–] = 7.0 × 10^–4, [H⁺] = 3.0, and I = 4.0 mol dm^–3 at 20 °C.

Figure 3

Typical plot of the ionic strength dependence of the observed first-order rate constant. [CrO₄^2–] = 7.0 × 10^–4, [PEG] = 1.3 × 10^–1, [H⁺] = 3.0 mol dm^–3 at 20 °C.

Figure 4

Plots of [H⁺] [PEG]/k_obs against 1/[H⁺] at different hydrogen concentrations and various temperatures. [CrO₄^2–] = 7.0 × 10^–4, [PEG] =1.3 × 10^–1, and I = 4.0 mol dm^–3.

Scheme 1. Speculated Mechanism of Reduction of Potassium Chromate by Poly(ethylene glycol) in Aqueous Perchlorate Solutions

Figure 5

Plot of 1/k′_obs versus 1/[Ru (III)]. [CrO₄^2–] = 7.0 × 10^–4, [PEG] =1.3 × 10^–1, [H⁺] = 3.0, and I = 4.0 mol dm^–3 at 25 °C.

Figure 6

Spectral changes (250–800 nm) in the reduction of potassium chromate by poly(ethylene glycol) in aqueous perchloric acid. [CrO₄^2–] = 7.0 × 10^–4, [PEG] =1.3 × 10^–1, [H⁺] = 3.0, and I = 4.0 mol dm^–3 at 20 °C. (a) In absence of Ru(III), (b) after reaction completion, and (c) in presence of Ru(III).