Combining natural zeolite and electrocoagulation with different electrode materials – electrode surface analysis and Taguchi optimization

Abstract

The treatment of compost leachate by the combined use of natural zeolite and electrocoagulation in a single step (ECZ) using different metallic materials of carbon steel, aluminium alloy, and zinc as electrodes was investigated. The experiments were divided into two parts. In the first part, the nine experiments were carried out in an electrochemical batch-type reactor under different operating conditions planned according to Taguchi's L9 orthogonal array design. The electrode material (Fe, Al, Zn), initial pH (4, 6 and 8), the addition of NaCl electrolyte (0.5, 1, and 1.5 g/L), and addition of zeolite (15, 20 and 25 g/L) were chosen as controllable factors, and their impact on chemical oxygen demand (COD) and turbidity were studied. The results show that the highest removal for both COD and turbidity was obtained with the Al electrode at pH 4 and an electrolyte addition of 0.5 g/L NaCl and 15 g/L of zeolite. However, the pH and the addition of zeolite do not have the same effect on COD and turbidity removal. Microscopic analysis of the electrode surfaces shows pitting, general or uniform corrosion, and coverage with a layer of corrosion products depending on the experimental conditions. The order of anode consumption was Zn > Fe > Al. Cathode consumption was evident in the experiments with the Al electrode, while both cathode consumption and increase in cathode mass was observed in experiments with the Fe and Zn electrode. In the second part, the influence of the initial COD concentration of the compost leachates treatment by ECZ with aluminium alloy electrodes was studied under optimal condition previously defined. Higher initial COD concentrations result in steeper curves of COD removal during ECZ and to a higher settling rate.