Zinc release by humic and fulvic acid as influenced by pH, complexation and DOC sorption

The complexation properties of humic substances, the stability of the dissolved humic-metal complex, and the interaction of complex with other soil constituents determine the solubility and mobility of heavy metals. This study investigated the influence of humic and fulvic acid on the release of zinc of soils from Fethiye (Mediterranean Region); Samsun (Black Sea Region); and Izmir (Aegean Region). The time-dependent release of zinc from the soil was evaluated using plots of logarithm of qt/qs versus time. The kinetics of zinc release by humic acid was characterised by two pseudo-first-order reactions. The observed rate coefficients in our model were k1 = 4.2 × 10− 3 h− 1 and k2 = 5 × 10− 5 h− 1, which indicated to an initial fast reaction followed by a slower reaction. The release of zinc increased with increasing humic acid concentration representing an approach to plateau formation at the higher humic concentrations. The highest releases of 334.5, 350.8 and 369.4 μmol kg− 1 for Fethiye, Samsun and Izmir soils, respectively, were obtained for a humic acid concentration of 1000 mg L− 1. A different situation was observed in treatments with 150 mg L− 1 humic acid solutions, where the precipitative removal of humic acid–zinc complexes occurred at about pH 7. The interactive behaviour of humic acid–zinc complexes represented a similar reduction in the zinc concentrations in aqueous solution. A rise in the zinc load caused only a slight increase in release, which indicated that zinc load had little effect on release. Changes in soil suspension pH reflected higher zinc releases. An increase in the suspension pH to approximately 10.5 resulted in an increase from 209.4 μmol kg− 1 to 280.2 μmol kg− 1 (300 mg L− 1 humic acid), which was attributed to the involvement of other functional groups like phenolic groups in the process. On the other hand, decreasing the suspension pH to about 5 revealed to an increase from 89 μmol kg− 1 to 602.7 μmol kg− 1 (100 mg L− 1 humic acid), which was determined to be mainly the contribution of acidity. The influence of humic acid sorption on zinc release was investigated by evaluating sorption behaviour with respect to pH, salt, clay, oxide and organic matter contents of soils. Sorption of humic acid on soils presented a linear behaviour, and as the partition and distribution coefficients imply, was highest for Fethiye Soil, followed by Samsun and Izmir soils. Since this order reflected the reverse order of zinc release from soils, the degree of zinc release from soils was related to the degree of humic acid sorption. Thus, the greatest release of zinc from Izmir Soil was explained by the low humic acid sorption. The release of zinc from soil was lower for fulvic acid solutions, despite its higher acidic functional group content. For fulvic acid concentration of 300 mg L− 1, a zinc release of 46.7, 85.7 and 148.5 μmol kg− 1 was obtained for Fethiye, Samsun and Izmir soils, respectively. This was linked to the strength of bonds involved in the process, as shown by the stability constants of zinc humic or fulvic acid complexes.