Selective absorption of heavy metals by soil and humic acids at different pH levels

Abstract

Studying processes of sorption-desorption of heavy metals from soil and its individual components allow to predict long-term effects under conditions of multielement contamination. Cations of heavy metals are showing competitive relationship due to their specific adsorption by components of the soil absorption complex, in particular humic acids. Interaction of chernozem podzolized heavy loam, isolated preparation of humic acids and soil residue after its removal with the solution which contain sulphates of Zn, Cd, Ni, Co and Cu in equal ratio were simulated at different pH levels. The task of research was to compare selective absorption of some heavy metals in soil and humic acids at pH values from 3 to 9. The experiment was performed by mixing 10 g of soil material or 0.1 g of humic acid with 100 ml of buffer solution, adding 10 ml 0.01 n equal mixture of Cd, Zn, Ni, Co, Cu and 2-hour exposure. Humic acids were extracted from this soil by 0.1 n NaOH after decalcification using 0.05 n H2SO4. Crystalline Copper sulphate, Zinc, Nickel, Cobalt, Cadmium were used to prepare the solution level of heavy metals. Ammonium acetate-buffer solutions with different pH (3.0, 5.0, 7.0, 9.0) were obtained by varying the ratio CH3СOOH and NH4OH. The experiment showed that competitive relationship between heavy metal in soil and humic acids. Copper has the highest specificity adsorption, Cobalt – the smallest. Under acidic and strongly acidic reaction content of Zn, Cd, Ni in equilibrium solution is close enough. The absorption of copper was increased by 50 % from strongly acidic to neutral reaction. Under alkaline conditions Cobalt and Copper were practically absent in the solution. Has been found that the absorption of heavy metals by soil decreases in sequence: Cu, Zn > Cd, Ni > Co. The selectivity of the absorption of heavy metals by humic acids was less marked, but the ability to sorption has the similar sequence: Cu > Zn > Ni > Cd > Co. The residue of soil after removal of humic acids has a high affinity for copper ions, which was absorbed twice from each other metals. Has been proved experimentally that selective adsorption of heavy metals in soil significantly depends on the pH, decreasing under acidic and strongly acidic conditions where hydrogen is successfully competing with them for exchange places in soil absorption complex. Deviation from the equivalent absorption (20 %) of each heavy metal in soil accounted for 15.0–26.0 % in acidic pH and 1.6–45.4 % in alkaline pH. After the interaction of heavy metals with humic acids in the most acidic medium fluctuations content elements in the equilibrium solution was 15.7–25.3 %, while the highest pH 11.0–26.7 %. Overall, the decrease of sorption capacity for humic acids elements can be placed in the following order: Cu > Zn > Ni > Cd > Co. This sequence is saved by a narrow ratio solution: adsorbent. Thus, under condition of polyelement contamination migration of Cobalt in soil may be more intensive than Zink, Cadmium, Nickel and a lot more than Copper, especially in alkaline pH.