Influence of Hydrophobe Structure on Biodegradation Pathways of Nonionic Ethoxylates

The structure of the hydrophobe in alcohol ethoxylates (AE) and alkylphenol ethoxylates (APE) has a significant influence on the biodegradation pathways and biodegradation rates of these nonionic surfactants in bacterial inocula present in waste treatment plants. APE undergo initial microbial attack at the terminal part of the polyoxyethylene (POE) chain followed by a shortening of the POE chain to leave intermediates which are slower to biodegrade than intact APE. Ethoxylates of alcohols derived from propylene or butene oligomerization appear to degrade by a similar mechanistic pathway. In contrast, ethoxylates of alcohols derived from linear olefins undergo initial attack which results in scission of the linear alkyl chain from the polyoxyethylene chain with rapid biodegradation of the hydrophobe to C02 and H20 accompanied by slower degradation of the polyoxyethylene chain. The use of double radiolabeled nonionic surfactants has shed significant light on their biodegradation pathways by providing detailed information on degradation of the hydrophobic and hydrophilic moieties. Introduction Increasing quantities of surfactants are used in household products and such industrial and institutional applications as textile processing, pulp and paper deinking and deresination and hard surface cleaning.' Of the two largest classes of surfactants, anionics and nonionics, the latter have shown the fastest growth rate over the past twenty years. Such environmental issues as foaming and aquatic toxicity shown by all the major classes of surfactants may be faced by selecting those whose structural features permit rapid biodegradation to intermediates which have non-foaming and non-toxic properties under practical waste treatment conditions. In an era of increasing environmental awareness, a detailed understanding of biodegradation pathways is essential in developing an overview of the potential impact surfactants usage and treatment practices can have on aquatic life. Secondary waste treatment, employing aerobic biodegradation, is the most widely used approach to reducing the undesired properiies of organic materials, like surfactants, which enter domestic waste treatment plants. In this approach, bacteria provide enzymatic systems which utilize oxygen to convert organics to C02, H20 and cellular mass. The process is complex with biodegradation proceeding through a series of intermediates produced by specific enzymes. Biodegradation rate is dependent upon the structure of the organic material assuming sufficient oxygen and viable bacteria are present. Those molecules having essentially linear structural features generally biodegrade in sewage treatment plants considerably faster than those having substantially branched carbon skeletons. A thorough review of the effect of structure on surfactant biodegradation has been written by Swishere2 This paper will review the biodegradation of nonionic surfactants. The major focus will be on alcohol ethoxylates and alkylphenol ethoxylates the two largest volume nonionics. In this paper the effect of hydrophobe structure will be discussed, since hydrophobe structure is considered more critical than that of the hydrophile in biodegradability of the largest volume nonionics. The influence of the hydrophobe on the biodegradation pathway will be examined with an emphasis on the use of radiolabeled nonionics. Structural features of nonionic surfactants Figure 1 summarizes the significant structural features of four classes of nonionic surfactants discussed in this paper. These are simplified representations of commercial surfactants which contain varying hydrophobe and hydrophile chain lengths. The hydrophiles are represented as polyoxyethylene chains, although such hydrophiles as polyglucosides and polyoxypropylene/polyoxyethylene chains are used to a lesser extent. Linear Primary Alcohol Ethoxylates (LPAE) The hydrophobes of LPAE are made in several ways :