Leather tanneries hazardous waste dumping on open land in Pakistan

摘要

Bentonite has been studied as an adsorbent for Cr(Ⅲ) and Cr(Ⅵ) ions for its possible use for cleaning the waste effluents from the leather tanneries. The chemical and structural properties of bentonite were determined. The sorption of Cr(Ⅲ) and Cr(Ⅵ)from aqueous solutions on bentonite has been studied by batch technique. Distribution coefficients (K_d) were determined for the sorption systems at different temperatures. Sorption data has been interpreted in terms of Freundlich, Langmuir and Dubnin-Raduskevich equations. Thermodynamic parameters for the sorption systems have been determined at different temperatures. The sorption of Cr(Ⅲ)on bentonite is exothermic in nature while of Cr(Ⅵ) is an endothermic process. The heat of sorption, AH° values for Cr(Ⅲ)and Cr(Ⅵ) were -51.94 KJ/mol at 298 K and 5.619 KJ/mol at 293 K respectively. The mean free energy of sorption, E for Cr(Ⅵ) is 10 KJ/mol which shows that chromate ions from aqueous solutions having 10~(-5) to 10~(-3) M concentrations are predominantly sorbed on bentonite by an ion-exchange process. The sorption data for Cr(Ⅲ) do not fit in the D-R equation. Negative values of AG° show the spontaneity of the sorption processes; AG° values of Cr(Ⅲ) become less negative at higher temperatures which shows that sorption is less favoured at higher temperatures, while the increase in the negative values of AG° of for Cr(Ⅵ) with the increase in temperature indicates that sorption is favoured at higher temperatures.