Studies on the Fate of Nonionic Surfactant in the Soil Environment

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In order to clarify the fate of such nonionic surfactants as alkyl ethoxylates,alkylphenol ethoxylates in the soil environment,the biodegradation test with soil perfusion method was carried out.In the biodegradation test, residual surfactants in the perfused fluid were analysed by total organic carban (TOC).It was observed that TOC decreased sharply immediately after the initiation of perfusion because of the adsorption onto soil,and biodegradation occured after lag time about 40 days.These results show that the adsorption of nonionic surfactants on the soil plays an inportant role in the disappearance phenomena in the soil environment.Then,the adsorption behavior of nonylphenol ethoxylate(NPE,n=10,15,18,20)on soil and soil clays in aqueous solution was examined.Adsorption isotherms of NPE on such clay minerals as Kaolin,Cericite and Ca-bentonite could be divided into two distinct stages below CMC (critical micelle concentration),at which adsorption amount attained nearly constant values. Isotherms showed a Langmuir-type in a lower concentration range,and then adsorption amount increased with an increase in concentration.The adsorption amounts decreased with increasing number of ethylene oxides,n, attached to nonylphenol.These results suggest that in a lower concentration range molecules of NPE arrange themselves on the clay surface in such a way that the hydrophilic constituents,ethylene oxides,are parrallel to the surface,while in a higher concentration range aggregates of two or three dimentional micelles are formed on the surface by the hydrophobic interaction.The adsorption behavior of NPE on soil clay was similer to that on clay minerals.On the other hand,adsorption isotherms on humic soil obeyed the Freundrich equation at concentration lower than CMC.This experiment is continued to clarify the adsorption properties of nonionic surfactants on humic substances.