Extraction and separation of cationic surfactants from river sediments: Application to a spectrophotometric determination of cationic surfactant in an aquatic environment using membrane filters

The quantitative extraction of cationic surfactant (CS+) in river sediments was studied. Further, the developed method was applied to the spectrophoto metric determination of CS+ in urban river sediment samples by solid-phase extraction with membranes. A mixture of methanol and hydrochloric acid was proposed as an eluent. Dried sediment was digested in the eluent under ultr asonic irradiation. After elution, the eluent was evaporated to almost dryn ess. The residue was dissolved in a small volume of methanol and diluted to a certain volume with water. The pH of the solution was adjusted to 4 - 5 to separate iron and some other metals as precipitates of hydroxides. The s olution was passed through two-piled membranes: first glass-fiber and then polytetrafluoroethylene (PTFE) membranes. A small volume of methanol was pa ssed through the membranes to elute any CS+ retaining on the membranes. Aft er passing the methanol solution through a cationic exchange resin column, the retained CS+ was eluted with methanol containing a high concentration o f sodium chloride. Water, Bromophenol Blue (BPB) and hydrochloric acid were added to the solution. The solution was passed through a mixed cellulose e ster membrane filter to retain an ion associate of CS+. BPB-. The retained ion associate was dissolved in a small volume of N,N-dimethylformamide toge ther with the membrane filter, followed by the addition of triethanolamine to make the solution alkaline. The absorbance due to BPB2- was measured at 603 nm against a reagent blank. This method was applied to the determinatio n of CS+ in river water and sediment. A cationic surfactant in sediments at 10(-5) mol kg(-1) levels was detected with satisfactory precision. It was found that CS+ was about 500-fold enriched in the sediment from water at th e place where domestic wastewater was discharged.