Microplastics in Sludges and Soils: A Comprehensive Review on Distribution, Characteristics, and Effects

Microplastic contamination in terrestrial environments has risen significantly, far exceeding levels in marine environments. This shift underscores the concerning prevalence of microplastics (MPs) in sewage sludge and soil, raising environmental apprehensions. Microplastics from various sources accumulate in sewage systems, consequently, sewage sludge and soil have transformed into primary reservoirs of microplastic pollutants, capable of infiltrating aquatic ecosystems. While using sludge to enrich soil provides nutrients, it simultaneously introduces substantial microplastic content, posing environmental hazards. These microplastics can accumulate in the soil, altering its properties and potentially polluting deeper soil layers and groundwater, compounding environmental risks. This review scrutinizes the abundance, types, and shapes of microplastics in sewage sludge and soil, evaluating their impacts and suggesting future research directions. Statistical analysis reveals higher microplastic concentrations in sludge (271 Particles/kg dry weight) than in soil (34.6 Particles/kg). Strong correlations between microplastic concentrations in soil and sludge (R2 = 0.95) underscore the significant influence of sludge application on soil ecosystems. The p-value of 0.0001 indicates a significant correlation between MP amounts in soil and sludge, while the p-value of 0.47 suggests no significant association between MP concentrations in wastewater and sludge. Research confirms that microplastics influence sludge properties, microbial communities, and soil characteristics, contingent on microplastic attributes and soil conditions. Predominantly, microplastic shapes found in sludge and soil are fibers and fragments, often linked to agricultural fertilizer use. Microplastics detrimentally affect soil bulk density and aggregate stability, impairing soil structure and surface. Furthermore, their presence alters pollutant transport behavior in soil, emphasizing the imperative to investigate microplastics' effects and transport mechanisms for mitigating environmental and health risks.