pH influence on selectivity and retention of heavy metals in some clay soils

Abstract

The problem of varying species of heavy metals existing as individual species or in combination with each other in simulated contaminant leachates (solutions) is studied in relation to selectivity and retention in four different kinds of soil. The influence of pH on the retention characteristics is also examined. The results obtained from the heavy metals single-species and composite-species adsorption tests indicate that in evaluating the attenuation potential of the soil substrate (i.e., contaminant attenuation), account needs to be given to the sensitivity of the soil – heavy metals contaminant system to the pH of the leachate and the competitive effect between the heavy metals present in the leachate. A change in the soil solution pH results in a corresponding change of the dominant retention mechanism of heavy metals in the soils. At high pH values, precipitation mechanisms (e.g., precipitating as hydroxides and (or) as carbonates) dominate. As pH decreases, precipitation becomes less important, and cation exchange becomes dominant. High amounts of heavy metals are retained in the soils if the soil buffer capacity remains high enough to resist a change in pH. The selectivity order of heavy-metal retention in soils depends on the pH of soil solution. At soil solution pH values above 4 or 5, when precipitation prevails, the selectivity order obtained is given as Pb > Cu > Zn ≥ Cd, as demonstrated for the illite, montmorillonite, and natural clay soils. At lower soil solution pH values, the selectivity order obtained is given as Pb > Cd > Zn > Cu, as can be seen in the case of kaolinite and montmorillonite. Key words : heavy metals, retention, selectivity, preferential adsorption, precipitation.

Sequential sorption of lead and cadmium in three tropical soils

“…Extracts were combined and stored at 4 � C prior to analysis of ex changeable Pb (E1Pb), with concentration being corrected for occluded/pore water E1Pb. The amount of sorbed Pb that was not exchanged by the NaNO 3 washes was operationally defined as being specifically sorbed or sorbed to high affinity sites (Yong and Phadungchewit, 1993). The I of the soil solutions was readjusted to 0.01 M via two 30 mL washes with 0.01 M NaNO 3 , shaking for 1 h during each rinse.…”

“…Thus, coulombic-type interactions are favored with other charged species or surfaces for Ca 2þ and Cd 2þ . Echeverria et al (1998) and Yong and Phadungchewit (1993) reported on the competitive sorption of various heavy metals in mineral and soil systems. They reported decreased sorption of Cd, Cu, Pb, and Zn when these metals were added together compared to when they were added alone to soils.…”

Application of the buffer theory for evaluating attenuation and natural remediation of ionic pollutants in aquatic ecosystems

“…The authors (Yong et al 1990) proved that high Pb uptake by the clays could affect the buffer capacity and the Pb retention capacity. In the paper (Yong and Phadungchewit 1993), it has been demonstrated that high amounts of heavy metals are retained in the soils if the soil buffer capacity remains high enough to resist a change in pH. The results from the selective sequential extraction analysis of heavy metals supported also the conclusion of the significance of soil buffer capacity with regard to heavy-metal retention (Yong and Phadungchewit 1993).…”

“…In the paper (Yong and Phadungchewit 1993), it has been demonstrated that high amounts of heavy metals are retained in the soils if the soil buffer capacity remains high enough to resist a change in pH. The results from the selective sequential extraction analysis of heavy metals supported also the conclusion of the significance of soil buffer capacity with regard to heavy-metal retention (Yong and Phadungchewit 1993). The capacity of the soils to retain high amounts of heavy metals as they receive increasing amounts of acid (i.e., as the pH is reduced) depended directly on the soil initial pH values and on their buffer capacities.…”

Selectivities of Potassium‐Calcium and Potassium‐Lead Exchange in Two Tropical Soils

“…A cation with high selectivity for a particular soil surface (exchanger) will be preferentially adsorbed relative to one with lower selectivity. Many studies, considering sorption of heavy metals (i.e., Pb, Cu, Zn, Cd, and Ni) added to soils and pure minerals (i.e., goethite, alumina, kaolinite, and zeolite), have inferred higher Pb selectivity relative to other heavy metals based on differences in total amount of metals sorbed or desorbed (Ahmed et al, 1998;Gao et al, 1997;Pardo, 2000;Yong and Phadungchewit, 1993). These studies did not focus on the exchangeable Pb fraction.…”