Effects of oxalic acid on sorption of Cd on a clay fraction of a soil

The mobility, bioavailability and environmental fate of heavy metal ions in soil are controlled by sorption onto soil minerals and solid organic matter. The sorption is strongly affected by the presence of various low-molecular-weight organic acids. The main objective of this study was to investigate the effects of oxalic acid on sorption of Cd on clay fraction of a soil.
The sorption of Cd on clay was studied as a function of pH and time of equilibrium (Cd concentration: 0.045 mM) in the range of 6-9.5, and as a function of metal concentration (Cd concentration in the range of 0.013-0.089 mM) in the absence and presence of oxalic acid with a fixed concentration of 500 μM at room temperature (23 ± 2 °C). The amounts of Cd sorbed on the adsorbent were calculated from the difference between the metal concentration in the blank and equilibrium concentration of Cd in the solutions.
The results of pH-dependent experiments showed that the oxalic acid decreased sorption of Cd onto the sorbent at all solution pHs. Experimental and modeling data from our kinetic reveal that the pseudo-second-order kinetic model gave the best fit. The Langmuir sorption model describes the interaction between Cd and the clay materials better than the Freundlich model. The value of Langmuir (KL) bonding constant was greater in suspensions without oxalic acid (26.6 Lg-1) as compared to suspensions containing this ligand (15.2 Lg-1). Also, the value of Freundlich (KF) bonding constants was greater in suspensions without oxalic acid (1.2 Lg-1) as compared to suspensions containing this ligand (1.8 Lg-1). These values are indication of low tendency of Cd for sorption on the clay surfaces in the presence of oxalic acid. In order to interpret the effect of oxalate on the sorption behavior of Cd onto the sorbent, knowledge of Cd speciation is essential. Species activities were estimated using the PHREEQC code. In the absence of oxalate, the predominant Cd(II) species is Cd2 over the pH range 3.0–9.0. In the presence of oxalate, Cd2 and Cd-Oxalate species were the major solution Cd(II) species at pH ≤10 . The formation of soluble complex of Cd-oxalate may be responsible for the inhibition of Cd sorption by the sorbent.
In general, our results demonstrate that the oxalate could have negative effects on Cd sorption to clay fraction of soils. Thus, it may play an important role in Cd availability and transport in soil.