Surfactant properties and tetrachloroethene (PCE) solubilisation ability of humic acid-like substances extracted from maize plant and from organic wastes: A comparative study

Humic acid-like substance (HA-like substance) extracted from maize plant residue at an yield of 81.1 ± 4.9 g kg^-1 of dry matter (dm) was tested for surfactant properties and ability to solubilise tetrachloroethene (PCE). Critical micelle concentration (CMC) of HA-like substance was detected to be 1986 mg L^-1. Both, HA-like substance composition and average molecular weight [MW] affected CMC and a multiple linear regression model was proposed: CMC = 12 246 - 56.19 alkyl-C - 0.532 MW (R² = 0.90; P <0.01, n = 7) where CMC was given in mg L^-1, alkyl-C was the percentage of total C, and MW was given in Da. Maize-HA-like substance solubilised PCE at the rate of 0.05 g of PCE for each gram of maize-HA-like substance, which was 3.6-9.6 times lower than the data obtained in our earlier work using HA extracted from organic wastes, but was higher than that obtained with commercial HA from leonardite. Taking into consideration the two-site model of solubilisation of PCE in surfactant (i.e., solute partitioning into the micellar hydrophobic core and dissolution and/or binding into the hydrophilic non-ionic shell) and macromolecular composition of HA-like substance, the non-ionic hydrophile-alkyl lipophile balance was expected to control PCE solubilisation as the good multiple linear regression indicated: log K_dom = -1.37 + 0.062 alkyl-C +0.055 O-alkyl-C (R² = 0.93, P <0.05, n = 6), where log K_dom represents the micelle-water partitioning of PCE (mL g^-1) and alkyl-C and O-alkyl-C represent the content of these two kinds of C detected by CP MAS ¹³C NMR (as % of the total C).