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

Laboratory study was performed to assess the impact of humification degree of humic acids isolated from composted rice straw (RS) and olive pomace (OP) on the metal loading capacity of extracted humic acids with Fe, Mn and Zn at 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 extracted from compost C. Both HA-c and FA-c realized more pronounced values of total carbon and nitrogen content. Adversely, HA-c and FA-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+2 reached to optimum values at pH 5 for all studied humic acids. While the maximum loading capacity of Mn+2 or Zn+2 was achieved at pH 6. In this respect, the maximum loading capacity of Fe+2, Mn+2 and Zn+2 were obtained for HA-c followed by HA-d