EFFECTS OF pH, IONIC STRENGTH AND TEMPERATURE ON THE RATE OF OXIDATION OF ARSENIC (III) BY DISSOLVED ORGANIC MATTER, DOM OBTAINED FROM SAWDUST, GROUNDNUT HUSK, AND RICE HUSK

Abstract

Objective: Industrial and many other processes produce waste streams that end in the environment and may contain toxic metals at concentrations above the permissible limits. Arsenic(III) is one of the most toxic ‘metal’ contaminants found in water and is well known to have adverse effects, such as skin cancer, etc. Thus, oxidation of arsenic(III) to the less toxic and less mobile arsenic(V) by dissolved organic matter (DOM) will help in controlling the environmental pollution by As(III).

Methods: Oxidation of Arsenic(III) ions by DOM obtained from Sawdust, DOM(SD), Groundnut Husk, DOM(GH) and Rice Husk, DOM(RH) was done at varying pH, ionic strength and temperature. These reactions rate were monitored using UV-visible spectrophotometry. DOM samples were confirmed by their UV-visible and infrared spectrophotometry.

Results; The reaction products were identified as  as it forms molybdenum blue salt {a product of ammonium molybdate(VI) with As(V) ions (in the environment of hydrazine sulphate)}, DOM samples were confirmed by their UV-visible absorption maxima having λ_max of 256 nm, 250 nm and 260 nm for DOM(GH), DOM(SD) and DOM(RH) respectively, and their infrared spectra with major absorption bands in the regions of 2500–3500 cm⁻¹ and 650-770 cm⁻¹ (O-H stretching and out of plane bending groups), 2500–3500 cm⁻¹, 1350–1470 cm⁻¹and 690–900 cm⁻¹ (C–H stretching, deformation, and bending and ring puckering respectively), 1650–1800 cm⁻¹ (C=O stretching of COOH), 1620–1680 cm⁻¹ (alkene/aromatic C=C stretching), 970–1250 cm⁻¹ (C–O stretching of alcohols/phenols) 880–995 cm⁻¹ and 1395–1440 cm⁻¹ (=C–H out of plane bending/C-O-H bending). The rate of oxidation of As(III) to As(V) by DOM increased with an increase in mass/volume percent of DOM {the increment was highest with DOM(SD)}, increased with an increase in pH and independent of the ionic strength. The reaction rate {oxidation of As(III) to As(V) by DOM} also increased with temperature but having a rate at 273 K comparable to that at 303-308 K.

Conclusion: It can be seen that DOM obtained from these wastes oxidised As(III) to As(V) as confirmed from the formation of molybdenum blue salt, and the spectroscopic analysis of the DOM.