Abstract
In the present study, an electrocoagulation process was applied to treat saline oilfield-produced water. The kinetics of simultaneous heavy metal and oil removal in the saline environment under different conditions including four-electrode materials of copper, zinc, iron, and aluminum, aeration and agitation rate, oil content, and salinity was investigated. The nature of the electro-generated species and possible abatement mechanisms were explored and compared by using FE-SEM/EDS, FTIR, XRD, and BET analyses. At low and high salinities, cadmium adsorption followed Langmuir and Freundlich models, suggesting the transformation of identical adsorption sites to heterogeneous ones. Cadmium removal efficiencies of 99/73% were obtained at low/high salinity with iron and 99.9 and 82% using copper and zinc electrodes in a saline environment. The cadmium adsorption capacity of different anode materials exhibited the order of copper > zinc > iron > aluminum. The adsorption capacity was considerably reduced in saline condition due to more crystalline structure and lower surface area and porosity of the particles while it was enhanced by the oil, caused by structural changes including more uniform pores, the elevated surface area, and porosity. The COD removal yield of 89% for low salinity and 80/73% at high salinity with/without aeration were achieved by iron. The highest COD removal yield of about 95% was achieved by the aluminum electrodes, compared to 85 and 87% for copper and zinc electrodes. The main removal mechanisms were outer- and inner-sphere complexation, and surface precipitation.
Highlights
Water supply challenges and the growing environmental concerns on wastewater discharge, make the two groups of hydrocarbons and heavy metals deserve more attention because of their high potential of toxicity and bioaccumulation[1]
(2) sum of errors squared (SSE), it can be inferred that the pseudo-second-order model correlates the obtained data the best, as shown in Supplementary
In which C0 and Ce represent initial and equilibrium cadmium concentrations respectively, V is the volume of Produced water (PW) in the EC reactor (L), m is mass of the electro-generated flocs (g), I is the electrical current (A), t is time (s), M is the molar mass of anode material, Z is the number of electrons transferred, and F depicts Faraday’s constant (C mol−1)
Summary
Water supply challenges and the growing environmental concerns on wastewater discharge, make the two groups of hydrocarbons and heavy metals deserve more attention because of their high potential of toxicity and bioaccumulation[1]. It was widely demonstrated that both pollutants pose fatal hazards to humans and the environment. Cadmium was proved to cause serious kidney, liver, and Wilson disease[2]. According to the recommendation of WHO for safe limits of Cd in wastewater and soil for agricultural purposes is 0.003 mg L−1 3. The treatment of wastewaters in terms of organic matter is of great importance because upon its discharge into the environment, it will cause serious harm by soil, surface and groundwater contamination, and health hazards[4,5]. The maximum allowable COD value for discharge into the environment is around 200 mg L−1 6 and the oil and grease of 10 mg L−1 7
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