(341b) Cost Effective Removal of Chemically Toxic Dyes Using Micro-Organism from Activated Sludge: Understanding Sorption Mechanism, Kinetics and Associated Thermodynamics

A systematic investigation was carried out for demonstrating the simple and cost effective removal of toxic dyes using activated sludge from municipal wastewater. The sorption process was found to follow Freundlich isotherm and pseudo 2^nd order rate kinetics. Difference in sorption efficiency for activated and deactivated sludge in aerobic and anaerobic conditions revealed the involvement of different micro-organism in sorption process with the indication of biodegradation. The sorption process was found to be spontaneous but entropy driven for RBBR and EBT while enthalpy driven for CR. The process was established to be a combination of adsorption as well as biodegradation.

With a push toward more inexpensive and eco-friendly options, biosorption and biodegradation are worthwhile methods of effluent treatment. Biosorption describes the uptake of inorganic or organic molecules through attachment to the cell membrane due to a number of chemical or physical processes on the surface of the microorganism. Biodegradation is a less commonly used method of removal as many synthetic dyes are difficult to degrade naturally or without producing any toxic byproducts as this process occurs when living organisms alter the molecular structure with the possibility of using it as a source of nutrients. Currently, treatment of dye contaminated water has been studied using organisms ranging from aquatic macrophytes, to eucalyptus bark, and even including potato peels and watermelon rinds as a desire to find affordable adsorbents is becoming more essential. One study used the fungus P. ostreatus supplemented with an alcohol to generate oxidizing laccases that could remove the color from solution through break down the dye Remazol Brilliant Blue R in solution.

Sewage sludge is a less commonly studied adsorbent for the removal of dyes yet it has several properties that make it conducive for the use of textile effluent treatment. However, coagulants widely used in the wastewater treatment are investigated for dyes adsorption. Wastewater sludge is s a combination of several viruses, bacteria, protozoa, and other small microorganisms that form aggregates in a suspended aqueous solution. The surfaces of these aggregates contain several functional groups that provide binding sites for adsorption of the dyes in solution. The functional groups of the chemical dyes interact with the active sites on the surface of the cell surfaces via different chemical or physical attraction methods including ion exchange, complexation, chelation, and micro-precipitation. Because wastewater sludge is commonly produced as a result of city sewage treatment it is easily acquired and particularly inexpensive.

This research proposes the use of locally obtained municipal wastewater sludge for the bio-removal of three synthetic textile dyes: Congo Red (CR), Eriocrhome Black T (EBT), and Remazol Brilliant Blue R (RBBR). Both anaerobic and aerobic samples of wastewater sludge were acquired and maintained for the duration of this research allowing for dual comparisons of adsorption between conditions for each of the dyes. The adsorption of the dyes on to the sludge samples were then analyzed to determine the kinetics and isotherm model fit for the data.

A systematic study was carried out to understand the effect of activation and deactivation of sludge on the sorption properties of the dyes. The % of sorption was found to decrease on deactivation which indicated that probably biodegradation also played significant role in the present sorption process. If the % of sorption is considered as combination of adsorption on the surface + the biodegradation, then on deactivation the % of sorption should decrease as there will be no contribution from biodegradation (considering surface area and surface charge remained unchanged on deactivation). Therefore, it is required to know how surface properties modified on deactivation to evident the significant contribution from biodegradation.

The difference in sorption behavior of the sludge obtained from aerobic and anaerobic conditions indicated that, probably the micro-organism involved in sorption process from both the sludge are appreciably different. An attempt was also made to understand that on deactivation, how the particle size of the sludge gets modified. For all the cases the size of the particles was found to be concentrated on ~ 0.12 μm. This study also gave us the indication that on deactivation, there was no significant change in the surface area of the sludge.

The adsorption of compounds onto the sludge which a combination of aggregated floc and suspended particles is one of the dominant removal pathway for dyes. Biodegradation is transforming substrate into new compounds through biochemical oxidation reactions inside the microorganisms with the presence and absence of dissolved oxygen. The disappearance of the dyes was monitored to evaluate their biodegradability within 240 hours of culture. RBBR and EBT have showed strong resistant to be biologically degraded by the microorganisms where slight decreasing took place during the first 100 h of incubation with 12.9 and 37.4 removal efficiency, respectively. Furthermore, the concentration of RBBR and EBT in the supernatant was approximately a straight line till the end of the test. CR have followed similar removal trend with other dyes, however, after 100 h of culture with the microorganism, its concentration kept declining over time which indicates that it has a higher biodegradation affinity to the microorganisms.

The negative value of the Gibb’s free energy for sorption primarily suggested that all the sorption processes are spontaneous. The enthalpy values associated with the sorption processes revealed that, the sorption of CR and EBT on the sludge is exothermic while that of RBBR is strongly endothermic. The release of the solvent molecules during the chemical interaction of the active coordinating sites of the sludge and the dye might be responsible for the enhancement in the entropy. This entropy driven process is also an indication of the formation of inner sphere complex between coordinating sites and the dye molecules. On the other hand, CR sorption was found to be largely driven by the enthalpy change.

The sorption of chemically toxic dyes on the micro-organism was demonstrated as an efficient and cost effective way of their removal. Analysis of different isotherm models revealed the predominance of Fruedlich isotherm with chemisorption. The sorption capacity for CR and EBT was found to be superior to that of other sorbent reported in the literature, while for RBBR it is comparable. The process was found to be energetically favorable following pseudo 2^nd order kinetics for all the dyes on activated sludge in aerobic and anaerobic conditions. The sorption of RBBR was found to be endothermic whereas for others they were exothermic. The sorption efficiency of sludge in aerobic condition was found to be more than that in anaerobic condition. The reduction in sorption on deactivating the sludge indicated that biodegradation is an additional channel for dye sorption or deactivation leads to the change in surface morphologies resulting the reduction. The size of the particle remained unchanged as the surface functional groups revealing biodegradation to be active during the sorption process.