Critical process parameters for optimum yield of Co3O4 catalyst for application in textile effluent treatment

Abstract

The interest in nanostructured materials has increased due to the electrical, catalytic, optimal and magnetic properties of the nanoscale particles that are uniquely different from those of their corresponding bulk counterparts. The technological applications of Co3O4 nanostructures such as solid-state sensors, anode materials in lithium-ion rechargeable batteries, electrochromic sensors, heterogeneous catalyst, energy storage and magnetic materials have drawn attention to the controlled synthesis of these nanomaterials. There is a lack in the understanding of the effect of synthesis operating conditions (such as reaction temperature, residence time, type of precursor anion and precursor concentration) and their interactions on the produced Co3O4 nanoparticles. During this study the produced Co3O4 nanostructures were applied as a catalyst for the degradation of methyl orange (MO) solution to establish if there is a relationship between the product yield, crystallite size and the degradation percentage. The chemicals that are used during the dying process as well as the structure of the dyestuff makes the generated effluent highly polluted and difficult to treat. Advantaged oxidation processes (AOP) have gained considerable interest due to the low selectivity to organic pollutants in wastewater. AOPs involve the generation of hydroxyl free radicals by using different oxidants that ultimately destroy components that are not removed under conventional oxidation processes. During this study, a Co3O4 nanocatalyst was produced through the hydrothermal synthesis using three precursor salts with different counter anions (cobalt acetate tetrahydrate, cobalt chloride and cobalt nitrate). A factorial design (face-centred central composite design) was applied to create a design that would produce a prediction model to evaluate the effect of the operating parameters (reaction temperature, precursor concentration and residence time) on the product yield and crystallite size. X-ray diffraction (XRD) analysis showed that after calcination Co3O4 nanostructures were produced. Comparing the maximum yield obtained for each precursor the order was as follows: Co(NO3)2 .6H2O (100%) > CoCl12 .6H6O (82%) > Co(C2H3O2)2 (66%). The product yields obtained for Co(NO3)2 .6H2O–derived Co3O4 ranged from 56% to 100%.The product yields obtained for CoC12 .6H2O–derived Co3O4 ranged from 1% to 82%. The product yields obtained for Co(C2H3O2)2–derived Co3O4 ranged from 12% to 66%. The crystallite size of Co(NO3)2 .6H2O– and Co(C2H3O2)2–precursor derived Co3O4 presented the same trends and ranged between 4.15 nm and 21.22 nm. For CoC12 .6H6O–derived Co3O4 nanoparticles, the crystallite sizes ranged between 3.06 nm and 17.97 nm. Methyl orange (MO) was treated using peroxymonosulfate that was activated by the synthesised heterogeneous Co3O4 nanocatalyst for 30 minutes. Comparing the efficiency of the Co3O4 nanocatalyst that was produced by three different precursor salts; it was found that Co(NO3)2 .6H2O –derived Co3O4 nanocatalyst performed the best, removing 96% of the MO solution while Co(C2H3O2)2–precursor derived Co3O4 nanocatalyst performed the worst, only removing 67.93%. A cost comparative analysis on the synthesis cost of Co3O4 via various cobalt precursors and treatment cost of MO solution was used to evaluate the viability of a full-scale treatment process. It was found that Co(NO3)2 .6H2O-derived Co3O4 nanocatalyst is the most cost efficient and had the best performance in the treatment of MO solution. The production process involves low reaction temperature and residence time to achieve the maximum yield whereas CoC12 .6H6O and Co(C2H3O2)2 precursor salts have the higher requirements for the process conditions.