Nickel adsorption in two Oxisols and an Alfisol as affected by pH, nature of the electrolyte, and ionic strength of soil solution

Abstract

The retention of potentially toxic metals in highly weathered soils can follow different pathways that variably affect their mobility and availability in the soil–water–plant system. This study aimed to evaluate the effects of pH, nature of electrolyte, and ionic strength of the solution on nickel (Ni) adsorption by two acric Oxisols and a less weathered Alfisol. The effect of pH on Ni adsorption was evaluated in surface and subsurface samples from a clayey textured Anionic ‘Rhodic’ Acrudox (RA), a sandy-clayey textured Anionic ‘Xantic’ Acrudox (XA), and a heavy clayey textured Rhodic Kandiudalf (RK). All soil samples were equilibrated with the same concentration of Ni solution (5.0 mg L−1) and two electrolyte solutions (CaCl2 or NaCl) with different ionic strengths (IS) (1.0, 0.1 and 0.01 mol L−1). The pH of each sample set varied from 3 to 10 in order to obtain sorption envelopes. Ni adsorption increased as the pH increased, reaching its maximum of nearly pH 6. The adsorption was highest in Alfisol, followed by RA and XA. Competition between Ni2+ and Ca2+ was higher than that between Ni2+ and Na+ in all soil samples, as shown by the higher percentage of Ni adsorption at pH 5. At pH values below the intersection point of the three ionic strength curves (zero point of salt effect), Ni adsorption was generally higher in the more concentrated solution (highest IS), probably due to the neutralization of positive charges of soil colloids by Cl− ions and consequent adsorption of Ni2+. Above this point, Ni adsorption was higher in the more diluted solution (lowest ionic strength), due to the higher negative potential at the colloid surfaces and the lower ionic competition for exchange sites in soil colloids. The effect of ionic strength was lower in the Oxisols than in the Alfisol. The main mechanism that controlled Ni adsorption in the soils was the ionic exchange, since the adsorption of ionic species varied according to the variation of pH values. The ionic competition revealed the importance of electrolyte composition and ionic strength on Ni adsorption in soils from the humid tropics. The presence of NaCl or CaCl2 in different ionic strengths affects the availability of heavy metals in contaminated soils. Therefore, the study of heavy metal dynamics in highly weathered soils must consider this behavior, especially in soils with large amounts of acric components.