Occurrence of herbicides and their transformation products in sewage sludge: a review

The beneficial reuse of sewage sludge in agricultural soils is limited by the accumulation of micropollutants of emerging concern, which may pose significant environmental and human health risks. This review summarises recent advances in understanding the occurrence, persistence, and fate of herbicides and their transformation products in sewage sludge. Data from various geographic regions are discussed, with a focus on implications for the safe reuse of biosolids in agriculture. Most available studies have been conducted in European Union countries, where land application of biosolids is a common practice. Twelve groups of herbicides and their transformation products have been identified in sewage sludge, including glyphosate and aminomethylphosphonic acid (AMPA), phenylureas, phenoxy acids, chloroacetamides, triazines and their metabolites, triazinones, phenylcarbamates, isoxazolidinones, benzoic acids, dinitroanilines, benzofurans, phenyl ethers, and other herbicides. Among these, triazines and their metabolites were the most frequently detected, with concentration ranges of 0.01-277 ng/g and not detected (n.d.)-237 ng/g, respectively. Glyphosate and AMPA were found at particularly high concentrations (n.d.-35 000 ng/g). Phenylurea herbicides (e.g., diuron and isoproturon) were detected in a limited number of studies, with concentrations ranging from not detected to 102 ng/g. Substantial concentrations of phenoxy herbicides (2,4-d, 2,4-db, and 2,4,5-t) were also reported in sewage sludge, ranging from 50.5-864 ng/g. The available scientific literature on the occurrence of herbicides in sewage sludge focuses mainly on older, often already banned compounds, while data on currently approved herbicides remain scarce. This review highlights the need for more comprehensive global assessments of herbicides and their transformation products in sewage sludge to ensure the safe agricultural use of biosolids and minimise risks to plants and other organisms. The current lack of systematic monitoring and documentation represents a critical knowledge gap in evaluating environmental exposure and associated risks.