Binding of cationic surfactants to humic substances

Commercial surfactants are introduced into the environment either through waste products or site-specific contamination. The amphiphilic nature of both surfactants and humic substances (HS) leads to their mutual attraction especially when surfactant and HS are oppositely charged. Binding of the cationic surfactants dodecyl-pyridinium chloride (DPC) and cetyl- or hexadecyl-pyridinium chloride (CPC) to purified Aldrich humic acid (PAHA), Dando humic acid (DHA), Inogashira humic acid (IHA), Laurentian fulvic acid (LFA) and Strichen Bs fulvic acid (SFA) is studied at pH 4.5–5 at 0.005 M NaCl. For PAHA CPC binding is also studied at pH 5 and 0.1 M NaCl. Measurements with the Mütek Particle Charge Detector (PCD) and poly-DADMAC, a strong cationic polyelectrolyte, are used to determine the charge of the HS samples. PCD measurements with the surfactants reveal that the surfactant–HS complexes reach their iso-electric-point (IEP) before the critical micelle concentration (CMC) is reached. At the IEP the adsorption values (mol/g) of CP+ and DP+ to PAHA are the same, i.e. at the IEP the charge associated with the HS is neutralized by bound surfactant ions. For the other humic acids (HAs) CPC binding at the CMC corresponds with the charge obtained with poly-DADMAC, but for the fulvic acid (FA) samples CP(C) adsorption at the IEP is larger than the FA charge. The surfactant–HS complexes flocculate around the IEP. Binding isotherms are obtained using surfactant electrodes. The results for CPC and DPC to the HA samples show a pseudo-plateau near the IEP, which is missing in the isotherms to the FA samples. The CP+–PAHA isotherms at 0.005 M and 0.1 M intersect at the IEP. The affinity of CP+ binding to PAHA is larger than that of DP+ due to the longer aliphatic tail of CPC. The bound amount of DP(C) decreases in the order PAHA ≫ IHA ≈ DHA ≫ LFA ≈ SFA. The results demonstrate that cationic surfactant binding to HS is due both to electrostatic and hydrophobic attraction and that the fate of HS in aqueous environmental systems can be strongly affected by cationic surfactants.