title: Adsorption of three azo reactive dyes by metal hydroxide sludge: effect of temperature, pH, and electrolytes.
abstract: Adsorption of azo reactive dyes by metal hydroxide sludge were investigated using CI Reactive Red 2 (RR-2), CI Reactive Red 120, (RR-120), and CI Reactive Red 141 (RR-141). The adsorption isotherms, including the Langmuir constants (Q degrees and b) and the Freundlich constant (K(f)), for RR-2 decreased with increasing temperature, but this was reversed for RR-120 and RR-141. This behavior implied an exothermic process for RR-2 but an endothermic process for RR-120 and RR-141. The enthalpy value of adsorption for RR-2, RR-120, and RR-141 was -5.56, 2.77, and 6.41 kJ/mol, respectively, indicating that the adsorption of the less charged dyes (RR-2) was mainly physical, but that of the more charged dyes (RR-120 and RR-141) was chemical. The optimum system pH of 8.6+/-0.3 was maintained even when the solution pH was varied from 3 to 10. Higher concentration or more valence of anions of electrolytes in dye solution caused decreasing dye adsorption efficiency of metal hydroxide sludge. A higher dosage of sludge is required for real textile wastewater (>1% w/v) than for the synthetic dye solution (0.2% w/v). The leachates of heavy metals from metal hydroxide sludge to the environment are very low, which are within the standard limit of industrial effluent and leachable substances.
authors.affiliation: Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, 10140 Bangkok, Thailand.
authors.forename: Suchapa
authors.initials: S
authors.lastname: Netpradit
authors.forename: Paitip
authors.initials: P
authors.lastname: Thiravetyan
authors.forename: Sirintornthep
authors.initials: S
authors.lastname: Towprayoon