Study On Preparation Of Activated Carbon From Textile Dyeing Sludge And Its Congo-red Adsorption Property

Textile and dyeing wastewater must be treated by physical-chemical and biological method to satisfy the emission control regulations. In this process, much printing and dyeing sludge was produced. With the development of printing and dyeing industry, the output of printing and dyeing sludge become larger and larger. The output of textile and dyeing sludge has close correlation with water quality of printing and dyeing wastewater and its treatment technics. The moisture of printing and dyeing sludge is very high, so the treatment of printing and dyeing sludge is very difficult. The safe disposal of sludge from textile dyeing industry requires research on bioavailability and concentration of heavy metals. There are few reports addressed heavy metals and their chemical fraction in textile dyeing sludge. A thorough understanding of the properties and toxicity of different types of textile dyeing sludge is necessary for the expansion of this field. Therefore, it has become an urgent issue to seek an effective treatment and disposal as well as comprehensive utilization method for textile dyeing sludge.In this study, concentrations and chemical speciation of heavy metals (Cd, Cr, Cu, Ni, Zn, Pb) in sludge from nine different textile dyeing plants were examined. Some physiochemical features of sludge from textile dyeing industry were determined, and a sequential extraction procedure recommended by the Community Bureau of Reference (BCR) was used to study the metal speciation. Cluster analysis and principal component analysis (PCA) were applied to provide additional information regarding differences in sludge composition. This research also use textile dyeing sludge (TDS) to prepare activated carbon, and optimized the condition for the activated carbon (AC) preparation from textile dyeing sludge with sawdust (TDSS). The effects of different preparation conditions on the final porous texture and yield were investigated. The effects of activation temperature, heating rate and NaOH/precursor ratio on the influence of iodine number were investigated. Based on the central composite design (CCD), quadratic model was developed to correlate the influential factors to the iodine number. At last, the TDSS-AC through optimum process conditions was used to remove Congo red in the water. The thermodynamics, kinetics and the mechanism of adsorption were studied.The results showed that Textile dyeing wastewater treatment process (biotreatment-flocculation or flocculation-biotreatment processing) is an important factor of sludge properties. Biotreatment-flocculation processing sludge presented higher organic matter content and HHV values of the sludge. Zn and Cu contents were the highest, followed by Ni, Cr, Cd and Pb. The concentration of Cd and Ni in some sludge samples exceeded the standard suggested for acidic soils in China (GB18918-2002). In sludge from textile dyeing plants, Pb, Cd and Cr were principally distributed in the oxidizable and residual fraction, Cu in the oxidizable fraction, Ni in all four fractions and Zn in the acid soluble/exchangeable and reducible fractions. The pH and heat-drying method affected the fractionation of heavy metals in sludge.The results of preparing TDSS-AC indicated that the activation temperature, the heating rate and the nitrogen flow rate had considerable impacts on the surface area, whereas particle size and grinding of precursor performed insignificant impacts. The textile dyeing sludge (TDS)/sawdust ratio of1:1was found to be the most cost effective. From the analysis of variance(ANOVA), the most influential factor on iodine number were sort the results followed by activation temperature, NaOH/precursor ratio and heating rate. The optimum conditions were as follows:temperature of739℃, heating rate of6.59℃/min and NaOH/precursor ratio of2.51g. Under the optimal preparation condition, the TDSS-AC was characteristic of a narrow micropore size distribution, and its iodine value and SBET were1518.89mg/g and1617.70m2/g, respectively. TDSS-AC was used to treat Congo red wastewater. The results indicated that the adsorption process can be well described by Langmuir isotherms, and the saturated adsorption capacity was31mg/g at50℃. The Congo red adsorption is an endothermic reaction and higher temperature is favorable for the adsorbed of Congo red. Pseudo-first-order model rate equation was found to interpret the kinetic data suitably.