Kinetics and isotherm analysis of 2,4-dichlorophenoxyl acetic acid adsorption onto soil components under oxic and anoxic conditions.

2,4-dichlorophenoxyl acetic acid (2,4-D, pKa = 2.8) is used extensively as a herbicide in agricultural practices. Its sorption behavior on both untreated and soils treated to significantly remove specific components (organic and iron and manganese [Fe-Mn] oxides and hydroxides phases) was investigated under oxic and anoxic conditions. The chemical and structural heterogeneity of the soil components were characterized by elemental analysis and X-ray diffraction (XRD). The coexistence of the various components seems to either mask sorption sites on the untreated soil surfaces or inhibit interlayer diffusion of 2,4-D. All sorption data conform to the Freundlich description and a pseudo-second-order kinetic model. There was a strong positive correlation between sorption capacity K(d), and surface area (r(2) ≤ 0.704), but a negative correlation was uncovered with both pH and organic carbon (r(2) ≤ -0.860). The results indicate that 2,4-D is preferably sorbed under oxic rather than anoxic conditions and it is greater on soils containing a high Fe content. There was incomplete 2,4-D sorption reversibility, with desorption occurring more rapidly under anoxic conditions. The study suggests that stimulation of Fe III reduction could be used for the bioremediation of a 2,4-D-contaminated site.

Snail shell as coagulant aid in the alum precipitation of malachite green from aqua system.

The ability of snail shell (SS) to act as coagulant aid in the alum precipitation of a basic dye (MG) was investigated. The proximate physicochemical characterization of the SS showed the pH(solution) to be 8.01, high fraction of the inorganic constituents (ash content=93.76%), the presence of Ca(2+) (99.74%) as the major metal ion present and the point zero charge (PZC) found at pH 7.9. The X-ray diffractometric analysis revealed the presence of aragonite. The stability and leaching of the SS, tested in different aqua medium (acidic, basic and neutral solutions) showed that the SS was less stable in the acidic medium. Both the alum and the SS were used, differently, for the dye precipitation. The alum alone had no precipitating effect on the MG dye molecules while SS alone was able to reduce the intensity of the dye. When the SS was used as coagulant aid in alum precipitation, the percentage of the MG molecule removed was enhanced. The effects of some process variables (coagulant/coagulant aid dosage, pH and flocculation time) were optimized by method of continuous variation. The optimum pH for the MG removal was found to range between 4 and 5 but the amount of MG removed was appreciable at all the pH studied. Studies on the effect of time on the flocculation of the precipitated MG molecule showed that the problem of redispersion and restabilisation encountered in alum precipitation could be overcome using alum-SS combination. The settling characteristics of the sludge obtained from the use of SS alone and alum-SS combination was studied by measuring the sludge volume index (SVI, mg/g) over time. The value of the SVI (mg/g) showed that the sludge produced from the alum-SS combination had better settling characteristics than the sludge got from the use of SS alone.