STUDIES ON SIMULTANEOUS ADSORPTION AND BIODEGRADATION OF CYANIDES IN INDUSTRIAL WASTEWATER

Abstract

Cyanide compounds are strictly regulated compounds worldwide because of their extreme toxicity. These are found in a number of foods and plants and are produced by certain bacteria, fungi, and algae and numerous species of plants and foods such as beans, almonds and cashew nuts etc, but their large scale presence in environment is attributed to the human activities. Although notorious for its poisonous characteristics, cyanide is one of the most indispensable industrial chemicals and is extensively used in various industries. Cyanide compound are produced as waste with varying concentrations in effluents from industries like, Metal plating, Mining, Coke plant, Paint and Ink formulation, Petroleum refining, Explosive manufacturing, Case hardening, Automobile manufacturing, Printed circuit board manufacturing, Chemicals, Pesticides industries and Synthetic fiber production units etc. Cyanide's strong affinity to metal ions makes it favorable agent in electroplating/ metal plating and mining (extraction of gold, silver etc.) industries. Hence, it is produced in large volumes from these industries. Cyanide is produced as wastes from these industries, along with ammonia, phenolic compounds, suspended particles, organic wastes and significant amount of heavy metals like copper, nickel, zinc, silver, iron etc. Cyanide forms complex compounds of varying toxicity and stability in the presence of these metal ions. The release of cyanide from these industries worldwide has been estimated to be more than 14 million kg/yr. The cyanide compounds are characterized by CN" functional group, which can take different forms. Generally cyanide compounds are present in environmental matrices and waste streams as simple cyanides (free, soluble, insoluble), complex cyanides (week, moderately strong, strong), cyanates and nitriles. Various treatment processes used for removal and/ or recovery of cyanide compounds include alkaline breakpoint chlorination, INCO process (by S02/ air), copper-catalyzed hydrogen peroxide, Caro's acid, natural attenuation, cyanide recovery, ozonation, electrolytic oxidation, ion exchange, acidification, AVR (acidification, volatilization and reneutralization) process, lime-sulfur, reverse osmosis, activated carbon adsorption, thermal hydrolysis and biological treatments etc. Although the physical and chemical processes are frequently used for cyanide removal, biological methods are gaining support as potentially inexpensive and environmental friendly alternatives. However, biological processes may not be sufficient for removal of cyanide compounds to the desired limit of 0.2 mg/L and therefore, polishing stage treatment is required. Among the other treatment methods, adsorption had been found to be most efficient for the removal of cyanide compounds and used extensively in mining industries. However, adsorption may not be used for highly concentrated cyanide bearing wastewaters and may be used as a polishing stage treatment process. Biodegradation of cyanide compounds by various microorganisms has been reported. But most of the reports deal with the study on utilization of cyanide as carbon and nitrogen sources. Although, many attempts have been made for biological treatment of low concentrated simple cyanide compounds in batch reactors, but reports on immobilized cell technology and biodegradation of metal complexed cyanides are very few. Biodegradation is performed in presence of microbes either in mobilized or immobilized phase. Recently developed methods dealing with the removal of various compounds are adsorption and biological treatment, either operated separately or simultaneously in one unit. The immobilization of living microbial cells on a suitable adsorbent improves the removal efficiency and process performance. This n improvement is due to the bio-layer formation on the adsorbent bed where adsorption and biodegradation occurs simultaneously. Unlike adsorption, there is a continuous diffusion of adsorbate onto the solid surface and back diffusion of solute into the solution phase. The solute remaining in solution exists in dynamic equilibrium with that of in the surface of bio film. Adsorption and the biodegradation successfully supplement each other in the various schemes of wastewater treatment. Microbial mass can, to some extent, adsorb the substances, but at the same time it also degrades them. On the other hand, adsorption of the substances onto adsorbent reduced the inhibitory effect of the substances for microbial mass. Accordingly, simultaneous adsorption and biodegradation (SAB) is expected to be more stable and the toxic compounds may be converted into less harmful substances. Granular/ powdered carbon is the most widely used adsorbent for cyanide removal. The surface chemistry of activated carbon and the chemical characteristics of adsorbate, such as polarity, ionic nature, functional groups, and solubility, determine the nature of bonding mechanisms as well as the extent and strength of adsorption. Granular activated carbon (GAC) has good adsorption capacity as well as bio layer formation capacity, however; the use of GAC for the immobilization of cells has rarely been reported for the removal of cyanide compounds. Although the fate of SAB for cyanide compounds on activated carbon is still not clear, but this process had been successfully used for many other chemicals and toxic substances. The present study has been undertaken with the objective to investigate the suitability if utilization of SAB process for removal of cyanide compounds from aqueous solutions and industrial wastewaters and to compare the results of SAB process with adsorption and biodegradation individually. Adsorption, Biodegradation and SAB process have been extensively studied for various microorganisms in batch in and continuous packed bed reactors with various cyanide compounds. Three types of cyanide compounds; sodium cyanide, zinc cyanide and iron cyanide, were used for preparation of stock cyanide solutions. 50-1000 mg/L of initial concentrations of cyanide was used to simulate the variable concentrations of cyanides found in various industrial wastewaters. Adsorption studies were carried out for studying the effects of various operating parameters like pH, adsorbent dose, size of adsorbent, temperature, contact time, and initial cyanide concentration on the removal of cyanides. The detailed characterization pertaining to physico-chemical, structural and morphological properties of GAC were carried out. Percentage removal of cyanide compounds increased with increase in adsorbent concentration, while removal per unit mass of adsorbent increased with the decrease in adsorbent concentration and equilibrium was attained more rapidly at lower concentrations. The adsorption of cyanide compounds was found to be pH specific. The optimum dose of GAC was decided from the study and used for all further studies. Although size of GAC particles did not show any significant effect on cyanide removal, but the optimum size was decided based on their particle characteristics. Diffusion of CN" from the bulk solution to the active surface sites appeared to be the slowest step as it took a very long time to reach the equilibrium condition. Thepercentage removal of cyanide compounds decreased with THE increase in initial concentration of cyanide. Biodegradation studies were carried out for studying the effectiveness of various microorganisms. Four pure cultures of Pseudomonas fluorescens, Pseudomonas putida, Stemphylium loti and Rhizopus oryzae were used for the study. Also a mixed culture of bacteria isolated from activated sludge was used for the present investigation. Biokinetic parameters (Maximum specific growth rate, umax; IV Monod constant, Km; Substrate inhibition constant, Kj and effectiveness factor, n) based on Monod model were estimated from the model equations for mixed cultures. The growth of microorganisms in the enrichment medium and medium with cyanide ions was observed by measuring cell density. The microorganisms adapted to grow at maximum cyanide concentrations were harvested and its ability to degrade cyanide was measured in biodegradation process. The effect of pH, temperature and initial concentrations on the removal efficiency were studied for the various microorganisms. All the microorganisms were found to have the maximum efficiency about neutral pH conditions and temperature range 25-30 °C (35 °C for mixed cultures). The growth of microorganisms was delayed in the presence of high concentrated cyanide solutions. It was found that cyanide was utilized as sole nitrogen source by microorganisms. Also, with the increase in initial concentration of cyanide, the percent removal decreased. SAB studies were conducted with optimum GAC dose added to biodegradation medium for immobilization of microorganisms on GAC surface. Optimum conditions as found in biodegradation and adsorption studies were maintained in SAB process for various microorganisms. In SAB process cyanide removal was started at an earlier time than that observed in biodegradation process. Due to the delayed growth of microbes in the culture media, adsorption would have occurred initially in the first phase followed by biodegradation. In SAB process equilibrium conditions were achieved faster as compared to adsorption. Although there is a decrease in percentage removal with increase in initial cyanide concentration, but SAB showed better performance in the removal efficiency for higher concentrations of cyanide than adsorption and biodegradation processes. Highest removal efficiency for metal cyanides for both biodegradation and SAB were found in Mixed cultures, but for NaCN Stemphylium loti showed better performance. Various steps involved in the adsorbate transport from the solution to the surface of the adsorbent particles have been dealt with by using the Weber-Moris plots versus XA for adsorption and SAB processes. The rate controlling parameters have been determined and it was found that macropore diffusion rate is larger than micropore diffusion rate. Lagergren equation was validated by plotting {1 - (Fraction of adsorbate adsorbed at time't'/Fraction of adsorbate adsorbed on the adsorbent under equilibrium)} versus time (t). Five isotherm equations: including the conventional Freundlich, Langmuir isothems, R-P have been fitted with the equilibrium adsorption and SAB data. Based on % relative absolute error of isotherms for various cyanide compounds and with various microorganisms immobilized on BAC the best fit isotherms have been decided. Fixed bed adsorption studies for plain and biologically activated GAC were carried out in two reactors each. Pseudomonas fluorescens, Stemphylium loti and mixed cultures were used separately for SAB study in the biologically activated GAC packed reactor-I. Based on the performance of reactor-I, reactor-II was operated with mixed culture immobilized GAC. Detailed experimental investigations were carried out with respect to the effect of immobilized cell, initial cyanide concentration and reactor design parameters. The reactors were packed with GAC particles (2-4 mm) and operated with initial cyanide concentrations of 50-500 mg/L. The effect of bed height and flow velocity on the breakthrough curve was studied. Column studies showed that breakthrough point depends on the bed height and flow rate. For higher concentrations of cyanide more contact time and/ or recirculation of the effluent was required. The SAB process was found to be feasible and effective for removal of cyanide and a pointer towards development of a new technology for cyanide removal.