Sorption of quaternary ammonium compounds to municipal sludge

The sorptive behavior of four quaternary ammonium compounds (QACs) - hexadecyl trimethyl ammonium chloride (C₁₆TMA), dodecyl trimethyl ammonium chloride (C₁₂TMA), hexadecyl benzyl dimethyl ammonium chloride (C₁₆BDMA), and dodecyl benzyl dimethyl ammonium chloride (C₁₂BDMA) - to municipal primary, waste activated, mesophilic digested, and thermophilic digested sludges was assessed at 22 °C. Batch adsorption of all four separately tested QACs to primary sludge reached equilibrium within 4 h. At a nominal, initial QAC concentration of 300 mg/L and a sludge volatile solids concentration of 1 g/L, the extent of adsorption was 13, 88, 67, and 89% for the C₁₂TMA, C₁₆TMA, C₁₂BDMA, and C₁₆BDMA, respectively, and correlated positively to the QAC hydrophobicity and negatively to their critical micelle concentration. Equilibrium partitioning data were described by the Freundlich isotherm model. The adsorption capacity of the four sludges was very similar. In binary QAC mixtures, QACs with relatively high adsorption affinity and at relatively high aqueous concentrations decreased the adsorption of QACs with a low adsorption affinity. At pH 7, about 40% of the sludge-C₁₂TMA desorbed, whereas less than 5% of the sludge-C₁₆BDMA desorbed in 10 days. The effect of pH was negligible on the desorption extent of C₁₂TMA at a pH range 4-10 over 10 days, whereas increasing the solution pH to 10 resulted in more than 50% desorption of C₁₆BDMA. Given the fact that approximately 50% of the municipal biosolids are land-applied in the US, the data of this study would help in the assessment of the fate of QACs and their potential effect on human and environmental health.