Ionic‐strength and pH effects on the sorption of cadmium and the surface charge of soils

Abstract

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp3 Ca(NO&, and various concentrations of NaN03 (0.01-1.5 mol dm-3) as background solutions at a range of pH values (3)(4)(5)(6)(7)(8).In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaN03 increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH > PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.

New insights into the sorption mechanism of cadmium on red mud

“…The ionic strength-dependent Cd 2þ sorption might be attributed to the weakly complexation of Cd like outer-sphere surface species on the red mud (Sparks, 1995;Goldberg and Johnston, 2001). The elevation of ionic strength could increase the net surface positive charges of the red mud at pH (6.5) below its PZC (8.5), which could drive the competition of Cd 2þ for the sorption sites and change the z potential in the sorption plane, thus decreasing the Cd 2þ sorption on the red mud (Naidu et al, 1994). Therefore, it can be presumed that Cd 2þ was sorbed probably through ion exchange reaction with the positive charges from exchangeable sites and formed outersphere complexes (Angove et al, 1997;Srivastava et al, 2005;Serrano et al, 2009).…”

“…Therefore, it can be presumed that Cd 2þ was sorbed probably through ion exchange reaction with the positive charges from exchangeable sites and formed outersphere complexes (Angove et al, 1997;Srivastava et al, 2005;Serrano et al, 2009). However, Naidu et al (1994) observed that Cd 2þ sorption was increased with the increase of ionic strength at pH below the PZC and attributed the phenomena to the fact that the specific sorption rather than simple ion exchange occurred in the sorption plane. Therefore, the unambiguous information about sorption mechanisms of Cd on red mud still needs to be supplied by other additional evidence.…”

The effect of ageing on the bioaccessibility and fractionation of cadmium in some typical soils of China

“…It was reported that the adsorption of Cd in the soil increased rapidly with increasing solution pH within the range of 2-5 (Naidu et al, 1994).…”

Mitigation of cadmium and arsenic in rice grain by applying different silicon fertilizers in contaminated fields

“…The NPK+ CaSi9000 treatment showed the greatest decrease in the rice Cd concentration (up to ∼73 %). These results were partially because of Cd immobilization caused by silicateinduced pH change in the soils, the increase of pH (Table 3), which altered the Cd distribution in soil fractions, reduced the phytoavailable Cd, and increased the allocation of metals into more stable fractions (Naidu et al 1994;Liang et al 2005;. This resulted in the observed significant decrease of Cd concentration in straw and grain.…”

“…The soil pH substantially affects the bioavailability of As and Cd and their absorption by plants (Reddy and Patrick 1977;Masscheleyn et al 1991;Marin et al 1993;Naidu et al 1994;Chan et al 2008). The soil was slightly acidic with pH 6.0 (Table 3).…”