Occurrence of the main metabolites of the most recurrent pharmaceuticals and personal care products in Mediterranean soils

Sulfonamide metabolites enhance resistance transmission via conjugative transfer pathways

Human beings release thousands of antibiotics into the environment, which could generate the related transformation products (TPs), most of which have yet to be identified and lack rigorous microbial risk information. This study aimed to investigate the impact and mechanisms of 4-nitro sulfamethoxazole, N4-acetylated sulfamethoxazole, and N4-acetylated sulfadiazine, three typical sulfonamide (SAs) metabolites, on the risk of antibiotic resistance genes (ARGs) transmission. The results revealed that TPs significantly enhance the risk of conjugative transfer of RP4 plasmid at clinically and environmentally relevant concentrations (10 ng/L to 100 μg/L), with a maximum increase of up to 73-fold. These three metabolites' capabilities to enhance the conjugative transfer of ARGs are more pronounced than the parent sulfonamides. The induction mechanisms of TPs on ARGs transmission are also more complex, which primarily arise from the enhancement of reactive oxygen species, further increased cell membrane permeability and upregulated bacterial secretion systems. Transcriptomic analysis validated the aforementioned biological processes and showed that TPs also increased the activity of toxin-antitoxin system and bacterial intracellular transposon, thereby promoting the spread of ARGs. This research contributes to a better understanding of the antibiotic-like effects of TPs, which is crucial for improving our understanding of non-antibiotic drug-induced bacterial resistance risks.

Enhanced activity of mixed-culture electroactive biofilms and sulfamethoxazole removal efficiency by adding N-acyl-homoserine lactones in bio-electrochemical system

The addition of exogenous quorum sensing signaling molecules significantly enhanced the degradation efficiency of antibiotics, such as chloramphenicol in bio-electrochemical systems (BESs). However, the effects and mechanisms by which AHLs addition in BES facilitated the removal of sulfamethoxazole (SMX) remained inadequately explored. This study systematically compared the electrochemical performance and SMX removal efficiency in BES under two conditions: with and without the addition of N-acyl-homoserine lactones (AHLs) signaling molecules. In comparison to the control group, the AHL-treated group exhibited an increase in maximum output voltage from 340 to 489.67 mV, alongside a notable enhancement in SMX removal efficiency over 120 h ranging from 14.65% to 15.76%. Analyses of the live and dead cells and extracellular polymeric substances (EPS) composition revealed that following AHLs addition, both the ratio of live to dead cells and protein content within EPS increased by 12.66% and 74.37%, respectively. Furthermore, microbial community structure analysis indicated that after AHLs supplementation, there was a marked increase in the abundance of electroactive microorganisms as well as antibiotic-degrading and nitrogen-removing bacteria. Notably, Klebsiella - characterised by its electroactivity along with antibiotic degradation and nitrogen removal capabilities - exhibited a relative abundance reaching 56.84% in AHL, reflecting an increase of 28.31% compared to Blank; additionally, electroactive bacteria Dysgonomonas showed a relative abundance rise of 2.49%. Collectively, these findings suggested that enhancements in SMX removal efficiency upon AHLs addition were primarily driven by improvements in electrochemical performance coupled with alterations in microbial community structure.Highlights The electrochemical performance in AHL was improved compared with Blank.The protein content in extracellular polymeric substances increased by 74.37% in AHL.The removal efficiency of sulfamethoxazole in 120 h increased by up to 15.76% in AHL.The abundance of functional bacteria such as Klebsiella increased in AHL.

Analysis of multi-class unregulated organic compounds in soil and biosolids using LC-MS/MS

Numerous unregulated organic compounds (UOCs) including pharmaceuticals, opioids, and personal care products (PCPs) end up in wastewater. UOC presence in biosolids (a wastewater treatment byproduct), which are applied to soil for different reasons raises environmental and health risk concerns. In this study, two multi-class extraction methods were developed and validated to target 111 UOCs from 8 different major families simultaneously in biosolids and biosolids-impacted soil. One method (M-SPE) is a modified version of EPA 1694, that uses triple solid-liquid extraction and solid phase extraction (SPE). The second method (EMR) is a super-fast method consisting in a single solvent extraction and EMR (enhanced matrix removal)-Lipid dispersive SPE. M-SPE performed better overall with 72 and 54 UOCs extracted with 50-130% recovery for soil and biosolids, respectively, compared to EMR for which only 49 and 43 UOCs achieved within the same range, respectively. EMR performed particularly well for the extraction of low concentration opioids from biosolids. The use of ENVI-Carb as an additional cleanup step and its potential to sorb analytes was also evaluated. Although >75% sorption of 27 UOCs occurred, ENVI-Carb (graphitized carbon) was needed to sufficiently clean extracts prior to injection to avoid precipitation and protect analytical systems. Application of these methods to environmental samples resulted in detection of some flame retardants, opioids, pharmaceuticals, PCPs and phthalates totaling 30 and 26 UOCs in biosolids and soil historically applied with biosolids, respectively. This methodology will be an asset to determining UOC concentrations in biosolids and biosolids-impacted soils.

How Pharmaceutical Residues Occur, Behave, and Affect the Soil Environment

Many pharmaceuticals (PhMs), compounds for the treatment or prevention of diseases in humans and animals, have been identified as pollutants of emerging concern (PECs) due to their wide environmental distribution and potential adverse impact on nontarget organisms and populations. They are often found at significant levels in soils due to the continuous release of effluent and sludge from wastewater treatment plants (WWTPs), the release of which occurs much faster than the removal of PhMs. Although they are generally present at low environmental concentrations, conventional wastewater treatment cannot successfully remove PhMs from influent streams or biosolids. In addition, the soil application of animal manure can result in the pollution of soil, surface water, and groundwater with PhMs through surface runoff and leaching. In arid and semiarid regions, irrigation with reclaimed wastewater and the soil application of biosolids are usual agricultural practices, resulting in the distribution of a wide number of PhMs in agricultural soils. The ability to accurately study the fate of PhMs in soils is critical for careful risk evaluation associated with wastewater reuse or biosolid return to the environment. The behavior and fate of PhMs in soils are determined by a number of processes, including adsorption/desorption (accumulation) to soil colloids, biotic (biodegradation) and abiotic (chemical and photochemical degradation) degradation, and transfer (movement) through the soil profile. The sorption/desorption of PhMs in soils is the main determinant of the amount of organic chemicals taken up by plant roots. The magnitude of this process depends on several factors, such as crop type, the physicochemical properties of the compound, environmental properties, and soil-plant characteristics. PhMs are assumed to be readily bioavailable in soil solutions for uptake by plants, and such solutions act as carriers to transport PhMs into plants. Determining microbial responses under exposure conditions can assist in elucidating the impact of PhMs on soil microbial activity and community size. For all of the above reasons, soil remediation is critical when soil pollutants threaten the environment.

Analytical Methods for the Determination of Pharmaceuticals and Personal Care Products in Solid and Liquid Environmental Matrices: A Review

Among the various compounds regarded as emerging contaminants (ECs), pharmaceuticals and personal care products (PPCPs) are of particular concern. Their continuous release into the environment has a negative global impact on human life. This review summarizes the sources, occurrence, persistence, consequences of exposure, and toxicity of PPCPs, and evaluates the various analytical methods used in the identification and quantification of PPCPs in a variety of solid and liquid environmental matrices. The current techniques of choice for the analysis of PPCPs are state-of-the-art liquid chromatography coupled to mass spectrometry (LC-MS) or tandem mass spectrometry (LC-MS2). However, the complexity of the environmental matrices and the trace levels of micropollutants necessitate the use of advanced sample treatments before these instrumental analyses. Solid-phase extraction (SPE) with different sorbents is now the predominant method used for the extraction of PPCPs from environmental samples. This review also addresses the ongoing analytical method challenges, including sample clean-up and matrix effects, focusing on the occurrence, sample preparation, and analytical methods presently available for the determination of environmental residues of PPCPs. Continuous development of innovative analytical methods is essential for overcoming existing limitations and ensuring the consistency and diversity of analytical methods used in investigations of environmental multi-class compounds.

Multiclass Analysis for the Determination of Pharmaceuticals and Their Main Metabolites in Leafy and Root Vegetables

The irrigation of soils with reclaimed contaminated wastewater or its amendment with sewage sludge contributes to the uptake of pharmaceuticals by vegetables growing in the soil. A multiresidue method has been devised to determine five pharmaceuticals and nine of their main metabolites in leafy and root vegetables. The method employs ultrasound-assisted extraction, clean-up via dispersive solid-phase extraction, and analysis through liquid chromatography-tandem mass spectrometry. Box-Behnken design was used to refine variables such as extraction solvent volume, time of extraction, number of extraction cycles, and the type and amount of d-SPE sorbent. The method achieved linearity (R2) greater than 0.994, precision (relative standard deviation) under 16% for most compounds, and detection limits ranging from 0.007 to 2.25 ng g-1 dry weight. This method was applied to a leafy vegetable (lettuce) and to a root vegetable (carrot) sourced from a local market. Parent compounds were detected at higher concentrations than their metabolites, with the exception of carbamazepine-10,11-epoxide.

Unveiling combined ecotoxicity: Interactions and impacts of engineered nanoparticles and PPCPs

Emerging contaminants such as engineered nanoparticles (ENPs), pharmaceuticals and personal care products (PPCPs) are of great concern because of their wide distribution and incomplete removal in conventional wastewater and soil treatment processes. The production and usage of ENPs and PPCPs inevitably result in their coexistence in different environmental media, thus posing various risks to organisms in aquatic and terrestrial ecosystems. However, the existing literature on the physicochemical interactions between ENPs and PPCPs and their effects on organisms is rather limited. Therefore, this paper summarized the ecotoxicity of combined ENPs and PPCPs by discussing: (1) the interactions between ENPs and PPCPs, including processes such as aggregation, adsorption, transformation, and desorption, considering the influence of environmental factors like pH, ionic strength, dissolved organic matter, and temperature; (2) the effects of these interactions on bioaccumulation, bioavailability and biotoxicity in organisms at different trophic levels; (3) the impacted of ENPs and PPCPs on cellular-level biological process. This review elucidated the potential ecological hazards associated with the interaction of ENPs and PPCPs, and serves as a foundation for future investigations into the ecotoxicity and mode of action of ENPs, PPCPs, and their co-occurring metabolites.

Emerging organic contaminants in the soil-plant-receptor continuum: transport, fate, health risks, and removal mechanisms

There is a lack of comprehensive reviews tracking emerging organic contaminants (EOCs) within the soil-plant continuum using the source-pathway-receptor-impact-mitigation (SPRIM) framework. Therefore, this review examines existing literature to gain insights into the occurrence, behaviour, fate, health hazards, and strategies for mitigating EOCs within the soil-plant system. EOCs identified in the soil-plant system encompass endocrine-disrupting chemicals, surfactants, pharmaceuticals, personal care products, plasticizers, gasoline additives, flame retardants, and per- and poly-fluoroalkyl substances (PFAS). Sources of EOCs in the soil-plant system include the land application of biosolids, wastewater, and solid wastes rich in EOCs. However, less-studied sources encompass plastics and atmospheric deposition. EOCs are transported from their sources to the soil-plant system and other receptors through human activities, wind-driven processes, and hydrological pathways. The behaviour, persistence, and fate of EOCs within the soil-plant system are discussed, including sorption, degradation, phase partitioning, (bio)transformation, biouptake, translocation, and bioaccumulation in plants. Factors governing the behaviour, persistence, and fate of EOCs in the soil-plant system include pH, redox potential, texture, temperature, and soil organic matter content. The review also discusses the environmental receptors of EOCs, including their exchange with other environmental compartments (aquatic and atmospheric), and interactions with soil organisms. The ecological health risks, human exposure via inhalation of particulate matter and consumption of contaminated food, and hazards associated with various EOCs in the soil-plant system are discussed. Various mitigation measures including removal technologies of EOCs in the soil are discussed. Finally, future research directions are presented.

Dissemination of nonsteroidal anti-inflammatory drugs (NSAIDs) and metabolites from wastewater treatment plant to soils and agricultural crops via real-scale different agronomic practices

One of the most consumed pharmaceutical subgroups across the world is nonsteroidal anti-inflammatory drugs (NSAIDs). However, the dissemination of these compounds to the natural environments through agronomic practices is a serious global problem. The hypothesis of this study is to reveal the transition of selected NSAIDs, paracetamol (PAR), diclofenac (DCF), ibuprofen (IBU), and naproxen (NAP) together with six main metabolites, detected in raw/treated wastewater (RWW/TWW) and sewage sludge generated in an urban wastewater treatment plant (WWTP) to soils and agricultural crops (corn, barley, sunflower, and sugar beet) through two widely applied agronomic practices, irrigation with TWW and application of sewage sludge as soil amendment. In other words, the cycles of 10 NSAIDs have been evaluated by simultaneously monitoring their concentrations in RWW/TWW, sewage sludge, soils, and crops. It was determined that the parent compounds and detected metabolites were treated at quite higher removal efficiencies (93.4 - >99.9%) in the studied WWTP, while DCF was eliminated poorly (7.9-52.2%). However, although it changes seasonally for some compounds, it was determined that the concentrations of almost all investigated NSAIDs increased at the determined irrigation points in the discharge channel (DC) where agricultural irrigations were performed. Apart from that, DCF, NAP, and 2-hydroxyibuprofen (2-OH-IBU) were always detected in sewage sludge seasonally up to about 20.5, 11.3, and 3.7 ng/g, respectively. While 2-OH-IBU was determined as the dominant metabolite in RWW, TWW, and sewage sludge, the metabolite of 1-hydroxyibuprofen (1-OH-IBU) was determined as the dominant compound in soils. Although 1-OH-IBU was not detected in TWW and sewage sludge in any season, detecting this metabolite as a common compound in all investigated soils (up to 60.1 ng/kg) reveals that this compound is the primary transformation product of IBU in soils. It was observed that at least one of the metabolites of IBU (1-OH-IBU and/or 2-OH-IBU) was detected in all plants grown (up to 0.75 ng/g), especially during the periods when both agricultural practices were applied. In addition, the detection of 1-OH-IBU with increasing concentrations from root to shoots in corn grown as a result of both agronomic practices shows that this compound has a high translocation potential in the corn plant. Apart from this, it was determined that PAR was detected in corn (up to 43.3 ng/kg) and barley (up to 16.8 ng/kg) within the scope of irrigation with TWW, and NAP was detected in sugar beet (up to 11.2 ng/kg) through sewage sludge application.

Dual role of soil-derived dissolved organic matter in the sulfamethoxazole oxidation by manganese dioxide

Manganese dioxide (MnO₂) can mediate organic pollutant oxidation in aquatic environments, which has been reported to be inhibited or promoted by dissolved organic matter (DOM) in different studies. It remains unresolved why conflicting results have been observed and whether such results depend on the type and concentration of DOM. Here, we used three types of well-characterized DOM derived from soil heated at 50, 250, or 400 °C (DOM_50, DOM_250, and DOM_400, respectively) to evaluate the impacts of DOM type and concentration and environmental pH on MnO₂-mediated oxidation of sulfamethoxazole, a widely detected and ecotoxic emerging pollutant. We observed that the degradation rate of sulfamethoxazole was possibly promoted by DOM_250 (pH 6‒8), while it was generally inhibited by DOM_50 and DOM_400. Furthermore, it was initially inhibited and then promoted with increasing DOM concentrations and was consistently less inhibited at a higher pH. The inter-DOM variations of sulfamethoxazole degradation could be explained by the more enriched polyphenolics in DOM_250 than in DOM_50 and DOM_400, whereas the weak promoting effect of DOM_400 indicates that high DOM aromaticity may not necessarily promote pollutant degradation. Our results reconcile the debate on the role of DOM in the oxidation of sulfamethoxazole by MnO₂ and highlight the decisiveness of the molecular composition and concentration of DOM and the reaction pH in the overall promoting or inhibiting role of DOM.

Road runoff as a significant nonpoint source of parabens and their metabolites in urban rivers

Parabens are widely added to food, cosmetics, and medicines as preservatives and are typical contaminants of pharmaceuticals and personal care products (PPCPs). However, their fate and transport in urban watersheds remain largely unexplored. This study investigated the role of road runoff as a critical nonpoint source of parabens and their metabolites in urban rivers based on 73 multimedia (road runoff and dust in different urban land uses, wastewater, stormwater discharge and river water) samples collected from a highly urbanized drainage area. Seven parabens and five metabolites were detected in the road runoff, with mean concentrations of ∑parabens and ∑metabolites equal to 47.5 ng/L and 4710 ng/L, respectively. The concentrations in road runoff were comparable to those in treated wastewater and river water and showed a land use pattern of residential > industrial > commercial. A first flush effect of the contaminants was observed in a heavy rainfall event with an antecedent dry period. In general, the population-based and area-based emission intensities of ∑parabens and ∑metabolites in road runoff were one order of magnitude higher than those in wastewater effluent during the rainfall events. This study provides quantitative evidence that road runoff can be a major pollution source of parabens and their metabolites in rapidly growing cities during the wet season and calls for the integrated management of nonpoint sources to prevent urban river contamination by typical PPCPs.

Soil and Water Management Factors That Affect Plant Uptake of Pharmaceuticals: A Case Study

Water and food security are of global concern. Improving knowledge on crops’ potential uptake of pharmaceutical compounds (PhCs) is necessary to guarantee consumer health and improve the public’s perception of reclaimed water reuse. This study aimed to determine how water management (bottom-up applied for being supplied by Subsurface Drip Irrigation) and the plant rhizosphere effect on the uptake of PhCs. Five PhCs were mixed: atenolol, carbamazepine, dicoflenac, ibuprofen and valsartan. A total of 5 treatments were considered: 3 concentrations of PhCs in agricultural volcanic soil: 0.1, 10 and 100 µg·L−1; 0.1 µg·L−1 in sterilized soil; and a blank with three plant replications at 30, 45, and 60 days after emerging. The maximum quantity of the added PhCs was 100 µg·kg soil−1. A variant of the QuEChERS method was followed to extract PhCs from samples. The limits of quantification were between 10 ng·L−1 and 100 ng·L−1 in extracts. No PhCs over the limits of detection were detected (0.06–0.6 µg·kg−1 of dry plant sample). Hence, the described water reuse methodology poses a negligible consumer risk, which contrasts with hydroponic systems in which this risk has been shown. The results are discussed in terms of the effects of irrigation system, water management and the soil-plant barrier.

Suspected-screening assessment of the occurrence of organic compounds in sewage sludge

The profiling of emerging organic pollutants present in sludge and generated during wastewater treatment is much more limited than in water. This is mainly due to the difficulty of sludge analysis because of its high content of organic matter and interfering compounds. In this study, a generic extraction method using a mixture of buffered water (pH 4.1) and solid phase extraction (SPE) clean-up was applied to samples of sludge obtained in different treatment plants. This extraction was followed by determination of the contaminants by ultra-high performance liquid chromatography coupled to high resolution mass spectrometry (UHPLC-HRMS), using suspected screening to detect the most relevant organic compounds that access the environment through sludge application. This screening (including >3000 substances, such as, pharmaceuticals, pesticides, metabolites and industrial chemicals) tentatively identified 122 compound and assigned most probable structure to 39. The set of compounds assigned to a probable structure was increased in 14 compounds by searching in a free database of metabolites. Fifteen compounds were unequivocally confirmed against the analytical standard. Pharmaceuticals and personal care products (PPCPs), with 31 substances identified and 8 confirmed were the main group of compounds. Compounds frequently detected in all sludge samples include nucleotides such as adenosine triphosphate, amino acids such as phenylalanine, or peptides such as leu-phe. Altogether, the results of this work highlight the interest of HRMS to draw the profile of organic compounds in complex matrices.

A novel multiscale biophysical model to predict the fate of ionizable compounds in the soil-plant continuum

Soil pollution from emerging contaminants poses a significant threat to water resources management and food production. The development of numerical models to describe the reactive transport of chemicals in both soil and plant is of paramount importance to elaborate mitigation strategies. To this aim, in the present study, a multiscale biophysical model is developed to predict the fate of ionizable compound in the soil-plant continuum. The modeling framework connects a multi-organelles model to describe processes at the cell level with a semi-mechanistic soil-plant model, which includes the widely used Richards-based solver, HYDRUS. A Bayesian probabilistic framework is used to calibrate and assess the capability of the model in reproducing the observations from an experiment on the translocation of five pharmaceuticals in green pea plants. Results show satisfactory fitting performance and limited predictive uncertainty. The subsequent validation with the cell model indicates that the estimated soil-plant parameters preserve a physically realistic meaning, and their calibrated values are comparable with the existing literature values, thus confirming the overall reliability of the analysis. Model results further suggest that pH conditions in both soil and xylem play a crucial role in the uptake and translocation of ionizable compounds.

Fate of sulfamethoxazole in compost, manure and soil amended with previously stored organic wastes

Application of organic wastes as soil fertilizers represents an important route of agricultural soil contamination by antibiotics such as sulfamethoxazole (SMX). Soil contamination may be influenced by the storage time of organic wastes before soil spreading. The objective of this work was to study the fate of SMX in two organic wastes, a co-compost of green waste and sewage sludge and a bovine manure, which were stored between 0 and 28 days, then incorporated in an agricultural soil that has never received organic waste and monitored for 28 days under laboratory conditions. Organic wastes were spiked with ¹⁴C-labelled SMX at two concentrations (4.77 and 48.03 mg kg^-1 dry organic waste). The fate of SMX in organic wastes and soil-organic waste mixtures was monitored through the distribution of radioactivity in the mineralised, available (2-hydroxypropyl-β-cyclodextrin extracts), extractable (acetonitrile extracts) and non-extractable fractions. SMX dissipation in organic wastes, although partial, was due to i) incomplete degradation, which led to the formation of metabolites detected by high performance liquid chromatography, ii) weak adsorption and iii) formation of non-extractable residues. Such processes varied with the organic wastes, the manure promoting non-extractable residues, and the compost leading to an increase in extractable and non-extractable residues. Short storage does not lead to complete SMX elimination; thus, environmental contamination may occur after incorporating organic wastes into soil. After addition of organic wastes to the soil, SMX residues in the available fraction decreased quickly and were transferred to the extractable and mostly non-extractable fractions. The fate of SMX in the soil also depended on the organic wastes and on the prior storage time for manure. However the fate of SMX in the organic wastes and soil-organic waste mixtures was independent on the initial spiked concentration.

Enhanced transformation of sulfamethoxazole by birnessite in the presence of gallic acid: Kinetics and pathways

The emergence of antibiotic agents like sulfamethoxazole (SMX) in soils and surface water can cause serious threat to human and animal health. In this work, mechanisms for the promotive effect of gallic acid (GA) on the transformation of SMX by birnessite were studied. In the absence of GA, the observed pseudo-first-order reaction constants (k_obs) decreased as the pH increased from 4.0 to 8.0, in agreement with the decrease in redox potential of birnessite with increasing pH. The changes in chemical state of surface Mn atoms suggested that Mn(IV) played a major role in SMX transformation. SMX was transformed mainly by the bond cleavage of SN and SC, NH oxidation, and self- or cross-coupling reactions, with the formation of seven transformation products. The presence of GA could significantly promote the transformation of SMX, which was more pronounced at pH 6.0 comparing to the effect observed at 4.0. This promotive effect was attributed to both the addition reactions between SMX and GA quinones and the condensation reactions between -COOH of quinones and -NH₂ of SMX or its transformation products. Moreover, the addition reaction between SMX and OH coming from the GA oxidation by birnessite was also proposed, while its contribution to SMX transformation was small. Furthermore, pyrogallol and methyl gallate that do not contain electron-withdrawing substituent like -COOH group are less effective in promoting SMX transformation than GA, suggesting the electron-density of β‑carbon is key to the occurrence of addition reaction. Our results demonstrate the important role of birnessite and naturally occurring phenolic acids in abiotic transformation SMX, which will profoundly affect its mobility and bioavailability in environments.

Pharmaceuticals and Their Main Metabolites in Treated Sewage Sludge and Sludge-Amended Soil: Availability and Sorption Behaviour

This work evaluated the availability and sorption behaviour of four pharmaceuticals and eight of their metabolites in sewage sludge and sludge-amended soil. Digested sludge and compost were evaluated. The highest levels found in digested sludge corresponded to caffeine (up to 115 ng g⁻¹ dm), ibuprofen (45 ng g⁻¹ dm) and carbamazepine (9.3 ng g⁻¹ dm). The concentrations measured in compost were even lower than in digested sludge. No compound was detected in sludge-amended soils. This fact could be due to the dilution effect after sludge application to soil. Different adsorption capacities in sludge–soil mixtures were measured for the studied compounds at the same spike concentration. In general, except for paraxanthine and 3-hydroxycarbamazepine, the metabolite concentrations measured in the mixtures were almost two-fold lower than those of their parent compounds, which can be explained by their mobility and lixiviation tendency. The log K_d ranged from −1.55 to 1.71 in sludge samples and from −0.29 to 1.18 in soil–sludge mixtures. The log K_d values calculated for compost were higher than those calculated for digested sludge. The obtained results implied that the higher organic carbon content of compost could influence soil contamination when it is applied to soil.