Efficiency of humic acid from various organic sources for reducing hexavalent chromium in aqueous solutions

In this study, efficiency of humic acids (HAs) derived from various sources to reduce CrVI in aqueous solutions was compared. HAs were extracted from leonardite, peat moss, peat, cocopeat, coal, common char, biochar, vermicompost and sewage sludge. Some chemical and spectral characteristics of the extracted HAs were measured. Then, the reducing efficiency of HAs was measured and its relationship with the determined properties was investigated. To measure the reducing efficiency of HAs, a concentration of 0.1 mM CrVI (as potassium dichromate) in a sodium nitrate solution (0.03M) with a pH of 2 and in the presence of 100 mg of HA per liter was used. The experiment was conducted in three ways (symbolized by E1, E2 and E3). They differ from each other with respect to the method of phosphate buffer addition to release CrVI ions adsorbed by HA. This buffer was added to an aliquot of final extract, to the whole volume of final suspension and to initial solution containing CrVI in experiments E1, E2 and E3, respectively. According to the results, the CrVI reducing efficiency depends not only on the nature of HA but on the method of experiment. The minimum reducing efficiency was observed for common char using experiment E2 and the maximum value was for biochar and cocopeat in all three experiments. The results showed that the two factors of ΔlogK and maximum reducing efficiency in the format of a multiple regression had a significant relationship with CrVI reducing efficiency of HAs.