Competitive adsorption-desorption reactions of two hazardous heavy metals in contaminated soils

Abstract

Investigating the interactions of heavy metals is imperative for sustaining environment and human health. Among those, Cd is toxic for organisms at any concentration. While Ni acts as a micronutrient at very low concentration but is hazardous toxic above certain threshold value. In this study, the chemical adsorption and desorption reactions of Ni and Cd in contaminated soils were investigated in both single and binary ion systems. Both Ni and Cd experimental data demonstrated Langmuir type adsorption. In the competitive systems, an antagonistic effect was observed, implying that both ions compete for same type of adsorption sites. Adverse effect of Cd on Ni adsorption was slightly stronger than that of opposite system, consistent with adsorption isotherms in single ion systems. Variation in ionic strength indicated that Ca, a much weaker adsorbate, could also compete with Cd and Ni for adsorption on soil particles. Desorption data indicated that Cd and Ni are adsorbed very tightly such that after four successive desorption steps, less than 0.5 % of initially adsorbed ions released into the soil solution. This implies that Ca, at concentration in equilibrium with calcite mineral, cannot adequately compete with and replace adsorbed Ni and Cd ions. This adsorption behavior was led to considerable hysteresis between adsorption and desorption in both single and binary ion systems. In the binary ion systems, desorption of Cd and Ni was increased by increase in both equilibrium concentration of adsorbed ion and concentration of competitor ion. The overall results obtained in this research indicate that Cd and Ni are strongly adsorbed in calcareous soil and Ca, the major dissolved ion, insignificantly influences metal ions adsorption. Consequently, the contaminated soils by Ni and Cd can simultaneously be remediated by environmentally oriented technologies such as phytoremediation.

Predicting Heavy Metal Adsorption on Soil with Machine Learning and Mapping Global Distribution of Soil Adsorption Capacities

“…Studies have widely reported that at certain pH, higher ionic strength is beneficial for the adsorption, while at other pH conditions, this effect may be reversed. − Such a pH may be near the pHzpc (pH of zero point of charge) of the soil . For example, at pH greater than pHzpc, increasing ionic strength decreased Cd adsorption regardless of the soil type because the cations competed with Cd for the active adsorption sites . However, at pH higher than a certain level, increasing ionic strength increased the adsorption of As­(V), and vice versa .…”

“…67 For example, at pH greater than pHzpc, increasing ionic strength decreased Cd adsorption regardless of the soil type because the cations competed with Cd for the active adsorption sites. 71 However, at pH higher than a certain level, increasing ionic strength increased the adsorption of As(V), and vice versa. 70 This is because the number of cations in the adsorption plane increases with increasing ionic strength, resulting in a less negative potential at the adsorption plane and thus facilitating the adsorption of As(V) by the soil.…”

Study on the Nickel Adsorption Characteristics of Lake Sediments and Soil

“…The sediment contains more organic matter than the soil, so it has a stronger adsorption ability to nickel and a stronger fixation ability, leading to less desorption. Low Ni desorption may be the buffer of sediment and soil against heavy metal pollution ( Davari et al, 2015). The desorption process can be regarded as the reverse process of soil adsorption process, and all factors affecting adsorption affect the desorption.…”

Chitosan-g-poly(acrylic acid)-bentonite composite: a potential immobilizing agent of heavy metals in soil

“…Contrarily, owing to their metal chelating functional groups, the organic desorbing agents were able to remove up to 90% of the adsorbed metals. Hence, either EDTA or DTPA could be utilized for regenerating the composite when applied for water purification (Ngah et al 2013;Davari et al 2015;Luo et al 2015). On the other hand, a lower desorption of adsorbed metals from the composite in the presence of Ca(NO3)2 makes the material suitable for immobilizing metals in contaminated soils where an environmental electrolyte concentration always exists.…”