Copper binding by olive mill solid waste and its organic matter fractions

This study investigated Cu binding by olive mill solid waste (OMSW), an organic amendment rich in soluble compounds, and its effect on metal adsorption by a mineral soil. Copper-binding curves were performed for OMSW suspensions, its filtrate and for water-soluble (WS), fulvic acid-like (FA) and humic-like (HA) fractions at constant pH and ionic strength. The ‘free copper’ concentration at equilibrium was measured by means of an ion-selective electrode (ISE). The results revealed that the water-soluble organic matter, which accounted greater ratio than humic-like fractions, was responsible for the majority of the Cu bound by the residue suspensions. Regarding the OMSW-fractions Cu-binding curves and the obtained conditional binding parameters (Langmuir–Freundlich isotherm), the humic-like fractions showed behaviour similar to that of soil humic fractions; except that the maximum binding capacity of HA (0.25 mol kg− 1) was significantly lower due to its lower content in acidic complexing groups (0.96 mol kg− 1 carboxylic type). In contrast to the heterogeneous nature of humic-like material, the WS fraction seemed to have a homogeneous distribution of Cu-binding sites which reflected the values close to unity taken by the heterogeneity parameter (m = 0.90). Despite that WS fraction has metal-binding affinity value (logKWS = 5.3) practically equal to the mean values determined for the FA and HA (log KFA = log KHA = 5.2), and that its maximum complexing capacity (0.46 mol kg− 1) is of the same order of magnitude as FA (0.54 mol kg− 1), it was able to bind much less Cu at lower concentrations. In incubation experiments with a loamy, mineral soil, the OMSW amendment (2% rate) slightly increased the soil Cu retention capacity with no significant change in soil pH. The increase of dissolved organic carbon (3.2–20.5 mg l− 1) resulting from OMSW application had a noticeable effect on Cu speciation in the soil solution only at lower metal concentrations in the soil solution (approximately < 3 × 10− 5 M).