Characterization and metal loading capacity of humic acids derived from composted rice straw and olive pomace affected by the humification degree

Abstract

The humification degree is a valuable characteristic of humic acids that indicates the maturity and intensity of aromatic rings, important not only for the maturation of composted materials but also for the ability to interact with metal ions. This investigation was conducted to extract humic acids from composted rice straw (RS) and olive pomace (OP) and determine their characteristics(total acidity, carboxylic groups and phenolic groups, elemental analysis and IR absorption). As well as, to assess the impact of humification degree on the metal loading capacity of isolated humic acids i.e.HA-a (60% RS+10% OP), HA-b(50% RS+20% OP), HA-c (40% RS+30% OP) and HA-d (cotton stalks and rice straw, farmyard manure). For this purpose, a laboratory study was performed to assess the metal loading capacity of extracted humic acids with Fe⁺², Zn⁺², and Mn⁺²at different pH values (4, 5, and 6). The results revealed that the highest values of total acidity, carboxyl, and phenolic-OH groups were achieved by HA-c. HA-c realized more pronounced values of total carbon and nitrogen content. Adversely, HA-c achieved less pronounced values of oxygen and hydrogen as well as H/C, O/C, and C/N atomic ratios. The loading capacity of Fe⁺²reached optimum values at pH 5 for all studied humic acids. While the maximum loading capacity of Zn⁺²orMn⁺²was achieved at pH 6. In this respect, the maximum loading capacity of Fe⁺², Zn⁺², and Mn⁺²was obtained for HA-c followed by HAd. These results seem to confirm that the humification degree governs the loading capacity of humic acids for Fe²⁺, Zn⁺², and Mn⁺²which plays an important role in regulating their bioavailability in soil.