Determination of insoluble soap in agricultural soil and sewage sludge samples by liquid chromatography with ultraviolet detection

Abstract

We have developed a new analytical procedure for determining insoluble Ca and Mg fatty acid salts (soaps) in agricultural soil and sewage sludge samples. The number of analytical methodologies that focus in the determination of insoluble soap salts in different environmental compartments is very limited. In this work, we propose a methodology that involves a sample clean-up step with petroleum ether to remove soluble salts and a conversion of Ca and Mg insoluble salts into soluble potassium salts using tripotassium ethylenediaminetetraacetate salt and potassium carbonate, followed by the extraction of analytes from the samples using microwave-assisted extraction with methanol. An improved esterification procedure using 2,4-dibromoacetophenone before the liquid chromatography with ultraviolet detection analysis also has been developed. The absence of matrix effect was demonstrated with two fatty acid Ca salts that are not commercial and are never detected in natural samples (C₁₃:₀ and C₁₇:₀). Therefore, it was possible to evaluate the matrix effect because both standards have similar environmental behavior (adsorption and precipitation) to commercial soaps (C₁₀:₀) to C₁₈:₀). We also studied the effect of the different variables on the clean-up, the conversion of Ca soap, and the extraction and derivatization procedures. The quantification limits found ranged from 0.4 to 0.8 mg/kg. The proposed method was satisfactorily applied for the development of a study on soap behavior in agricultural soil and sewage sludge samples.

Component analysis of fat, oil and grease in wastewater: challenges and opportunities

“…The establishments that consume or produce fats/oils high in palmitic and oleic acids are more prone to higher FOG deposit formation and accumulation, while those that consume or produce fats/oils high in lauric or linoleic acids are less prone. Cantarero et al 21 described a different method to measure the concentration of Ca salt in FOG. They tested ethyl acetate, acetone, water and methanol, all of which extracted the analytes to varying degrees, with methanol showing the highest recovery rate of 95%.…”

“…Fatty acids with longer carbon chains tend to be more insoluble in water. 21 Furthermore, the toxicity of a FA salt is largely influenced by the length of its alkyl chain, increasing with length of alkali chain. Hence, C12:0, a FA salt with a 12-carbon chain, is more toxic than C10:0, a FA salt with a 10-carbon chain.…”

“…This is because longer alkyl chains make the compound more hydrophobic, or water-repellent, and less likely to dissolve in water. 21…”