Adsorption And Oxidation Approach In Coking Wastewater Treatment: Material Synthesis And Process Optimization

Abstract

Coking wastewater originating during the coke production process in steel industry is considered to be one of the most polluting sources for industrial pollution due to presence of toxic contaminants. These toxic contaminants comprise of different compounds such as phenolic compounds, polycyclic aromatic hydrocarbons, cyanide, ammonia, thiocyanate, and heavy metals (chromium, mercury etc.). In view of this problem, coking wastewater was collected and characterized to determine the toxicity levels and contaminants of problematic concern. Phenol, organics and chromium were selected as the target pollutants whose removal have been addressed in different phases of this research work. Low cost waste materials of environment friendly nature such as eggshells and chitosan has been used in synthesis of adsorbents to investigate its adsorption efficiency for target pollutants present in coking wastewater. Chitosan based adsorbents in the form of chitosan flakes, chitosan crosslinked tripolyphosphate (CS/TPP) and magnetic chitosan beads (MCBs) were investigated in batch sorption tests and all proved successful in adsorption of phenol and organics from coking wastewater. Commercial mass production of chitosan flakes with its advantages of natural abundance, environment friendly nature, biodegradability, nontoxicity and efficiency to remove diverse pollutants makes it a promising alternative to activated carbon for treatment of coking wastewater. The modified chitosan (CS/TPP) proved to be much more efficient in adsorption of pollutants from coking wastewater. High adsorption performance, zero solubility, ease in separation and regeneration makes CS/TPP a potential candidate for compact small scale adsorption systems in treating coking wastewater. Nonlinear regression error analysis using error functions and sum of normalized errors (SNE) technique was adopted for optimization of isotherm parameters. Chitosan/calcined eggshell carbon (CS/CES) composite adsorbent was studied for the adsorptive removal of phenol in which taguchi experimental design was used for optimization of preparation process. Taguchi optimized preparation conditions of activation temperature (800℃), activation time (60 mins) and mix ratio (1:1) resulted in the maximum phenol adsorption capacity of 10.82 mg g-1. Coking wastewater was treated with activated carbon supported heterogeneous Fenton oxidation in a lab scale reactor. The process parameters were optimized using desirability approach based on response surface methodology and comparison was made between Fenton and photo Fenton in the oxidative treatment of wastewater. Simplified synthesis procedure, minimum iron leaching, less sludge production, low H2O2 consumption, lesser catalyst dose and treatment capabilities under wider pH conditions make the proposed treatment process economical and efficient. Finally, an environment friendly hybrid catalyst (CsFe@TPP/TiO2) was developed for the removal of toxic hexavalent chromium. The catalyst synthesis parameters and adsorption process were optimized using taguchi experimental design. Adsorption isotherm, kinetics, thermodynamics and characterization by FTIR and SEM analysis after adsorption were utilized for understanding the adsorption mechanisms. The catalyst maintained high removal efficiencies (around 80%) even after 5 reuse cycles with a maximum adsorption capacity of 82.89 mg g-1. Photocatalytic oxidative capabilities of the catalyst revealed good Cr (VI) removal under both visible (70.88%) and ultraviolet light (98.74%) in an irradiation time of 420 min.