Abstract


Activated carbon was prepared from dried municipal sewage sludge and batch mode adsorption experiments were conducted to study its potential to remove composite reactive dye from dyeing unit effluent. Adsorption parameters for the Langmuir and Freundlich isotherms were determined and the effects of effluent pH, adsorbent dosage, contact time and initial dye concentration were studied. The toxicity characteristic leaching protocol (TCLP) was used to assess the acceptability of sewage sludge derived activated carbon from the environmental point of view.


Keywords: Reactive dye, Sludge derived activated carbon, Commercial activated carbon, Adsorption isotherms, TCLP.

Introduction


Almost every industry uses dyestuffs to dye their products and the residual and unspent substances are usually discharged into the aquatic environment. Discharge of such effluents imparts color to the receiving water and interferes with its intended beneficial use. In addition, recent reports suggest that several color-causing substances are micro- toxic to aquatic biota (Lee et al., 1999; Kadirvelu et al., 2000; Kadirvelu et al., 2003).


Amongst the different industrial wastewaters with different types of color-causing substances, synthetic textile organic dye wastes occupy a prominent position. Over 7 x 105 t and approximately 10,000 different types of dyes and pigments are produced worldwide annually. It is estimated that 10% to 15% of the dye is lost in the effluent during the dyeing process (Young et al., 1997; Danis et al., 1998).


Increasingly stringent legislation on the decontamination of wastewater has created interest concerning the use of activated carbons for this purpose (Shukla et al., 2002; Chiou and Li, 2003; Garg et al., 2003; Ozacar and Sengil, 2003; Malik, 2003). However, activated carbons are expensive and therefore their use may imply carrying out regeneration and reactivation procedures (Rozada et al., 2003). In recent years, this has prompted growing research interest in the production of carbon-based adsorbents from a range of residues, mainly industrial or agri-cultural by-products. The production of activated carbon from residues helps in saving nonrenewable natural resources and producing a valuable product with potential applications in pollution control.


Reactive dyes are used extensively in the textile industry, due to their superior dyeing properties, especially in terms of fastness. Azo, Anthraquinone and Pthalocyanin are major classes of reactive dyes. The use of reactive dyes is increasing and was estimated to reach a market share of 50% in 2004 (Phillips, 1996). However, only a limited number of studies on the removal of reactive dyes have been found in the literature.


The focus of this research is the evaluation of the adsorption potential of sewage sludge derived activated carbon for composite reactive bearing wastewater and comparison with a commercial activated carbon as a reference. The effects of dose of solids loading, pH, contact time and initial dye concentration on the adsorption of composite reactive dye wastewater on to sludge derived and commercial activated carbon were investigated. The Langmuir and Freundlich isotherm models were used to represent the equilibrium data. The toxicity characteristics leaching protocol (TCLP) was used to determine the leachability of toxic elements from sludge derived activated carbon to assess its acceptability from the environmental point of view.