Primary Biodegradation of Commercial Fatty-Alcohol Ethoxylate Surfactants: Characteristic Parameters

Abstract

This paper concerns the primary biodegradation of different commercial fatty-alcohol ethoxylate surfactants (FAEs), applying the OECD 301 E test for ready biodegradability. Changes were made both in the carbonchain length of the surfactants as well as in the number of units of ethylene oxide (EO) in its molecule. The biodegradation were monitored, analysing the colony-forming units (CFU) formed during this process. From the biodegradation profiles drawn for the FAEs, parameters characteristic of the biodegradation process were defined: latency time (tL), biodegradability at 50 h of assay (B), half-life (t1/2), mean biodegradation rate until reaching biodegradability of 85% (VM), and the residual concentration of the surfactant (SR). The analysis of these parameters enabled the establishment of the influence of surfactant concentration and structure during the biodegradation process. The increase in the surfactant concentration lowered the rate of the biodegradation process and the biodegradability of the surfactant in addition to the half-life and residual concentration of the surfactant. The mean biodegradation rate, VM, for fatty-alcohol ethoxylates increased with the number of EO units and molecular weight of the surfactant. At low initial test concentrations (less than 25 mg/L), the concentration of the residual surfactant rapidly diminished with biodegradation time. For higher concentrations, after an adaptation period of the microorganisms, the surfactant concentration declined exponentially and the biodegradation rate became far slower for all the surfactants. The parameters characteristic of the growth curves: specific growth-rate, k, and the yield of biomass production per gram of surfactant, Yap, made possible the quantification and corroboration of the results during the biodegradation process.