Search

Environmental science & technology 56(23), 16546-16566 (2022). doi:10.1021/acs.est.2c02976, Published by American Chemical Society, Columbus, Ohio

The influence of an ionic functional group on the fate of chemicals in the environment, especially the formation of non-extractable residues (NER), has not been systematically investigated. Using 4-n-dodecylphenol[phenylring

Quantification of nonextractable residues (NER) of pesticides in soil is feasible by use of radioactively labeled compounds, but structural information on these long-term stabilized residues is usually lacking. Microorganisms incorporate parts of the radiolabeled ((14)C-) carbon from contaminants into microbial biomass, which after cell death enters soil organic matter, thus forming biogenic nonextractable residues (bioNER). The formation of bioNER is not yet determinable in environmental fate studies due to a lack of methodology. This paper focuses on the development of a feasible analytical method to quantify proteinaceous carbon, since proteins make up the largest mass portion of bacterial cells. The test substance (14)C-bromoxynil after 56 days forms more than 70% of NER in soil. For further characterization of NER the amino acids were extracted, purified, and separated by two-dimensional thin-layer chromatography (TLC). Visualization of the (14)C-amino acids was performed by bioimaging, unambiguous identification by GC-MS and LC-MS/MS. Our analysis revealed that after 56 days of incubation about 14.5% of the (14)C-label of bromoxynil was incorporated in amino acids. Extrapolating this content based on the amount of proteins in the biomass (55%), in total about 26% of the NER is accounted for by bioNER and thus is not environmentally relevant.

The nature of the abiotic birnessite (δ-MnO(2))-catalyzed transformation products of phenolic compounds in the presence of soil organic matter is crucial for understanding the fate and stability of ubiquitous phenolic carbon in the environment. (14)C-radioactive and (13)C-stable-isotope tracers were used to study the mineralization and transformation by δ-MnO(2) of two typical humus and lignin phenolic monomers--catechol and p-coumaric acid--in the presence and absence of agricultural and forest soil humic acids (HAs) at pH 5-8. Mineralization decreased with increasing solution pH, and catechol was markedly more mineralized than p-coumaric acid. In the presence of HAs, the mineralization was strongly reduced, and considerable amounts of phenolic residues were bound to the HAs, independent of the solution pH. The HA-bound residues were homogeneously distributed within the humic molecules, and most still contained the unchanged aromatic ring as revealed by (13)C NMR analysis, indicating that the residues were probably bound via ester or ether bonds. The study provides important information on δ-MnO(2) stimulation of phenolic carbon binding to humic substances and the molecular distribution and chemical structure of the bound residues, which is essential for understanding the environmental fates of both naturally occurring and anthropogenic phenolic compounds.

The long-term behavior of the herbicide atrazine and its metabolites in the environment is of continued interest in terms of risk assessment and soil quality monitoring. Aqueous desorption, detection, and quantification of atrazine and its metabolites from an agriculturally used soil were performed 22 years after the last atrazine application. A lysimeter soil containing long-term aged atrazine for20 years was subdivided into 10 and 5 cm layers (at the lysimeter bottom: soil 0-50 and 50-55 cm; fine gravel 55-60 cm depth, implemented for drainage purposes) to identify the qualitative and quantitative differences of aged (14)C-labeled atrazine residues depending on the soil profile and chemico-physical conditions of the individual soil layers. Deionized water was used for nonexhaustive cold water shaking extraction of the soil. With increasing soil depth, the amount of previously applied (14)C activity decreased significantly from 8.8% to 0.7% at 55-60 cm depth whereas the percentage of desorbed (14)C residues in each soil layer increased from 2% to 6% of the total (14)C activity in the sample. The only metabolite detectable by means of LC-MS/MS was 2-hydroxyatrazine while most of the residual (14)C activity was bound to the soil and was not desorbed. The amount of desorbed 2-hydroxyatrazine decreased with increasing soil depth from 21% to 10% of the total desorbed (14)C residue fraction. The amount of (14)C residues in the soil layers correlated well with the carbon content in the soil and in the aqueous soil extracts ( p value = 0.99 and 0.97, respectively), which may provide evidence of the binding behavior of the aged atrazine residues on soil carbon. The lowest coarse layer (55-60 cm) showed increased residual (14)C activity leading to the assumption that most (14)C residues were leached from the soil column over time.

Clay minerals in soils control the sorption and mobility of nitroaromatics in munitions contaminated soils. Therefore, effect of exchangeable cations (NH4+ , K+, Ca2+, and Al3+) on sorption-desorption of trinitrotoluene (TNT) was studied in two reference soils viz sandy loam and silty clay. Compared to control soils, K+ ion saturation significantly increased TNT sorption in both the soils, while other cations decreased TNT sorption. Effect of K+ saturation on sorption of TNT was more pronounced in organic matter poor sandy loam soil (seven times) then silty clay (two times). Desorption followed the reverse trend and greater amounts of sorbed TNT were retained in the K+ saturated soils.

There has been increasing interest in enhancing natural attenuation of munitions-contaminated soils. Present study reports the effect of increasing soil organic matter content on fate and mobility of trinitrotoluene (TNT) and metabolites in soil columns. This study was performed using 30-cm-long columns containing a top 5 cm of contaminated soil as a source layer and an uncontaminated soil (25 cm) adjusted to 0.5, 1.0, 1.5 and 3.0% organic carbon (OC) content using compost. Contaminated soil layer was fortified with uniformly ring-labeled (14)C-trinitrotoluene (TNT) or 2,4-dinitrotoluene (DNT); in total there were 8 treatments. Columns were leached with synthetic rain water under unsaturated flow conditions in downside up direction. There was significant increase in the retention of both (14)C-TNT and (14)C-DNT in soils with increasing soil OC content and in 3.0% soil OC content column1% TNT/DNT was recovered in the leachate. Further, degradation of TNT and metabolites from contaminated soil was significantly increased and resulted in greater soil-bound residues. Formation of monoamino-dinitrotoluene (ADNTs), diamino-mononitrotoluene (DANTs) and monoamino-mononitrotoluene (ANTs) metabolites was greatly enhanced with increase in OC content of soils. Study suggests that increasing OC content of contaminated soil to 3.0% significantly enhanced the reduction of nitroaromatics to more polar amine metabolites and the formation of soil-bound residues.

Phytoremediation, the use of plants to extract contaminants from soils and groundwater, is a promising approach for cleaning up soils contaminated with heavy metals. However its use is limited by the time required for plant growth, the nutrient supply and, moreover, by the limited metal uptake capacity. Synthetic chelators have shown positive effects in enhancing heavy metal extraction, but they have also revealed several negative side-effects. The objective of this study was to investigate the use of three natural low molecular weight organic acids (NLMWOA) (citric, oxalic, and tartaric acid) as an alternative to synthetic chelators. Slurry-, column-, toxicity- and phytoextraction experiments were performed. For the phytoextraction experiment the three NLMWOA were applied to a copper- and a lead-contaminated soil respectively. A significant increase in copper uptake was visible only in the citric acid treatment (67 mg kg-1) in comparison to the EDTA treatment (42 mg kg-1). The NLMWOA application showed no enhanced effect concerning the lead phytoextraction. A possible explanation for this lack of significance could be the rate of the degradation of NLMWOA. This rate might well be too high for these heavy metals with low mobility and bioavailability such as lead. The amounts of NLMWOA applied to the soil were very high (62.5 mmol kg-1 of soil) and the effect was too little. In this respect EDTA, which was applied in very small amounts (0.125 mmol kg-1) was more efficient. Thus making NLMWOA unsuitable to enhance phytoextraction of heavy metals from soil.

The sorption of organic contaminants in soil is mainly attributed to the soil organic matter (SOM) content. However, recent studies have highlighted the fact that it is not the total carbon content of the organic matter, but its chemical structure which have a profound effect on the sorption of organic contaminants. In the present study sorption of two nitroaromatic contaminants viz. trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT) was studied in different SOM fractions viz. a commercial humic acid, commercial lignin and humic acid and humin extracted from a compost. 13C-DP/MAS NMR studies indicated that the structural composition of the organic carbon in different SOM fractions was different. The order of sorption of the nitroaromatics in the different sorbents was: humic acid-commercial > humic acidcompost > humin about lignin. Among the aliphatic and aromatic carbon fractions (representing bulk of SOM matrix), adsorption parameter Kf(1/n) for nitroaromatics sorption correlated well with the aliphatic carbon (r = 0.791 for TNT and 0.829 for 2,4-DNT) than the aromatic carbon (r = 0.634 for TNT and r = 0.616 for 2,4-DNT). However, among carbon containing functional groups, carbonyl carbon showed strong positive correlation with sorption of TNT (r = 0.991) and 2,4-DNT (r = 0.967) while O-alkyl carbon showed negative correlation (r = 0.832 for TNT and r = 0.828 for 2,4-DNT). The study indicates that aliphatic domains in the SOM significantly affect the non-specific sorption of both the nitroaromatic contaminants.

Phytoextraction has revealed great potential, however it is limited by the fact that plants need time and nutrient supply and have a limited metal uptake capacity. Although the use of synthetic chelators, such as EDTA, enhances heavy metal extraction, it also produces the negative side effects of high phytotoxicity, as well as leaching of essential metals. The aim of this research was to investigate the application of wool, in mobilising metals and in improving the phytoextraction of metals-contaminated soil. We performed column experiments with 14 d and 7 d partially hydrolysed wool as chelating agent on a silty-loamy sand agricultural soil. In the column experiment the 14 d wool hydrolysate mobilised 68% of Cu in soil, whereas in the case of Cd it mobilised 5.5%. The model plant selected for the phytoextraction experiments was tobacco (Nicotiana tabacum). The plant uptake of Cd and Cu, assisted by the application of 6.6 g kg(-1) wool hydrolysate was increased by 30% in comparison to the control plants. The application of 13.3 g kg(-1) wool hydrolysate enhanced the Cu uptake by up to 850%. Moreover, high leaching probability frequently observed when applying chelating agents, such as EDTA or ethylene diamine disuccinate (EDDS), were not detected. The use of hydrolysed wool therefore merits further investigation.

Trinitrotoluene (TNT), a nitroaromatics, is a major pollutant in explosive contaminated soils. Present study reports the effect of initial concentration of TNT on its degradation kinetics in soils. Soils from two contaminated sites viz. Clausthal and Elsnig, Germany, were mixed with an uncontaminated reference soil to get different initial concentrations (mg/kg) viz Clausthal-1 (54.29), Clausthal-2 (30.86), Clausthal-3 (7.05) Elsnig-1 (879.67), Elsnig-2 (86.43); Elsnig-3 (8.16) and Elsnig-4 (0.99) and were spiked with ring UL-(14)C-TNT (100KBq/50g soil). Except Elsnig-1 and Elsnig-2 soils, TNT degraded at same rate in all the soils. Higher persistence of TNT in Elsnig-1 and Elsnig-2 soils appears to be due to higher initial concentrations of nitroaromatics which may be toxic to soil microorganisms. 2-Amino-4,6-dinitrotoluene (2-ADNT) and 4-amino-2,6-dinitrotoluene (4-ADNT) were recovered as major metabolites of TNT. Distribution of bound (14)C-activity in different soil organic matter (SOM) fractions (humic acid, fulvic acid and humin) was assayed by alkali extraction of solvent extracted Clausthal-1 and Elsnig-1 soils. Results indicate that humic acid accounted for maximum bound activity followed by fulvic acid and humin fractions. Expressing (14)C-activity bound/unit weight of organic carbon content of SOM fractions showed that 3 times more (14)C-activity was bound to Elsnig humic acid than Clausthal humic acid. Similarly, activity associated with Elsnig fulvic acid was 7 times higher than that of Clausthal fulvic acid suggesting that chemical nature of SOM fractions plays a significant role in binding of contaminants.

Cadmium poses a major environmental and human health threat because of its constant release through anthropogenic activities. A need, therefore, exists for cost-effective remediation procedures. Phytoremediation, the use of plants to extract contaminants from soils and groundwater, has revealed great potential. However, it is limited by the fact that plants need time, nutrient supply and, moreover, have a limited metal uptake capacity. Synthetic chelators have shown positive effects in enhancing heavy metal extraction through phytoremediation, but they have also revealed a vast number of negative side-effects. The objective of this research was to investigate the use of humic acids as an alternative to synthetic chelators. Humic acids were applied to a cadmium-contaminated soil at various dosages, and the uptake of cadmium into Nicotiana tabacum SR-1 was determined in relation to the amounts of total and bioavailable cadmium in the soil. It was found that the theoretical bioavailability of cadmium, as determined by diethylenetriaminepentaacetic acid (DTPA) extraction, did not change, but its plant uptake was enhanced significantly, in some cases up to 65%. Humic acids added at a rate of 2 g kg −1 soil increased the cadmium concentration in the shoots from 30.9 to 39.9 mg kg −1 . A possible reason for this enhancement is the decrease in pH, resulting in higher cadmium availability. Another possibility taken into account is that plants may take up cadmium complexes with humic acid fragments, which result from microbiological degradation or, self-dissociation.