Manjia Chen

Arsenic Concentrations in Florida Surface Soils

Background As concentrations in soils are important for defining whether a soil is polluted. Arsenic concentrations in 441 taxonomically and geographically representative surface soils were determined using EPA Method 3052 (HCl-HNO 3 -HF digestion). Cumulative distribution plots indicate that As concentrations follow a log-normal distribution and depend on soil type. Sample geometric mean(GM) (the exponential mean of the log-transformed distribution) As concentrations (mg kg -1 ) generally follow the soil taxonomic order of Histosols (2.35) > Inceptisols (0.98), Mollisols (0.72) ≥ Ultisols (0.51)≥ Alfisols (0.39), and Entisols (0.36) > Spodosols (0.18). The highest As concentrations were found in soils that occur exclusively or prevalently in wetlands, such as Hemists (3.16-9.44), Saprists (0.15-11.7), Aquents (0.10-50.6), Aquolls (0.03-3.34), and Aquepts (0.03-38.2). Both linear and multiple regressions indicate soil properties (clay, pH, cation-exchange capacity [CEC], organic C, and total Al), especially total Fe and P, are important factors affecting natural background concentrations of As in Florida soils. Arsenic release from bedrock (limestone) and As bioaccumulation by aquatic organisms are possible explanations for relatively high As in those wetland soils, The use of a single regulatory value criterion for As contamination in soil cannot provide an adequate assessment given the natural variation in soil As. Baseline soil-As concentration, which was defined as 95% of the expected range of background As concentrations in different soil categories, is necessary for properly assessing potential As contamination.

Anaerobic Transformation of DDT Related to Iron(III) Reduction and Microbial Community Structure in Paddy Soils

We studied the mechanisms of microbial transformation in functional bacteria on 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) in two different field soils, Haiyan (HY) and Chenghai (CH). The results showed that microbial activities had a steady dechlorination effect on DDT and its metabolites (DDx). Adding lactate or glucose as carbon sources increased the amount of Desulfuromonas, Sedimentibacter, and Clostridium bacteria, which led to an increase in adsorbed Fe(II) and resulted in increased DDT transformation rates. The electron shuttle of anthraquinone-2,6-disulfonic disodium salt resulted in an increase in the negative potential of soil by mediating the electron transfer from the bacteria to the DDT. Moreover, the DDT-degrading bacteria in the CH soil were more abundant than those in the HY soil, which led to higher DDT transformation rates in the CH soil. The most stable compound of DDx was 1,1-dichloro-2,2-bis(p-chloro-phenyl)ethane, which also was the major dechlorination metabolite of DDT, and 1-chloro-2,2-bis-(p-chlorophenyl)ethane and 4,4-dichlorobenzo-phenone were found to be the terminal metabolites in the anaerobic soils.

Effect of nitrate addition on reductive transformation of pentachlorophenol in paddy soil in relation to iron(III) reduction.

Reductive dechlorination is a crucial pathway for anaerobic biodegradation of highly chlorinated organic contaminants. Under an anoxic environment, reductive dechlorination of organic contaminants can be affected by many redox processes such as nitrate reduction and iron reduction. In the present study, batch incubation experiments were conducted to investigate the effect of nitrate addition on reductive dechlorination of PCP in paddy soil with consideration of iron transformation. Study results demonstrate that low concentrations (0, 0.5 and 1xa0mM) of nitrate addition can enhance the reductive dechlorination of PCP and Fe(III) reduction, while high concentrations (5, 10, 20 and 30xa0mM) of nitrate addition caused the contrary. Significant positive correlations between PCP degradation rates and the formation rates of dissolved Fe(II) (pearson correlation coefficients rxa0=xa00.965) and HCl-extractable Fe(II) (rxa0=xa00.921) suggested that Fe(III) reduction may enhance PCP dechlorination. Furthermore, consistent variation trends of PCP degradation and the abundances of the genus Comamonas, capable of Fe(III) reduction coupled to reductive dechlorination, and of the genus Dehalobacter indicated the occurrence of microbial community variation induced by nitrate addition as a response to PCP dechlorination.

The key microorganisms for anaerobic degradation of pentachlorophenol in paddy soil as revealed by stable isotope probing

Pentachlorophenol (PCP) is a common residual persistent pesticide in paddy soil and has resulted in harmful effect on soil ecosystem. The anaerobic microbial transformation of PCP, therefore, has been received much attentions, especially the functional microbial communities for the reductive transformation. However, the key functional microorganisms for PCP mineralization in the paddy soil still remain unknown. In this work, DNA-based stable isotope probing (SIP) was applied to explore the key microorganisms responsible for PCP mineralization in paddy soil. The SIP results indicated that the dominant bacteria responsible for PCP biodegradation belonged to the genus Dechloromonas of the class β-Proteobacteria. In addition, the increased production of (13)CH4 and (13)CO2 indicated that the addition of lactate enhanced the rate of biodegradation and mineralization of PCP. Two archaea classified as the genera of Methanosaeta and Methanocella of class Methanobacteria were enriched in the heavy fraction when with lactate, whereas no archaea was detected in the absence of lactate. These findings provide direct evidence for the species of bacteria and archaea responsible for anaerobic PCP or its breakdown products mineralization and reveal a new insight into the microorganisms linked with PCP degradation in paddy soil.

Effects of the Fe II /Cu II Interaction on Copper Aging Enhancement and Pentachlorophenol Reductive Transformation in Paddy Soil

The present study investigated copper aging and pentachlorophenol (PCP) reductive transformation under the effects of the Fe(II)/Cu(II) interaction in paddy soil in south China. Kinetic measurements demonstrated that the PCP reductive transformation rate (k) could be promoted in the presence of no more than 0.375 mM Cu(II) and inhibited in the presence of no less than 0.5 mM Cu(II). The highest k value in the presence of 0.25 mM Cu(II) corresponds to the lowest redox potential (E(p)) value of active Fe species. The increasing trend in E(p) of the active Fe species is consistent with the declining trend in the k value of PCP reduction and vice versa. Dissolved Cu(II) is gradually transformed into immobilized Cu species during PCP reduction, which indicates that Cu aging is enhanced by the Fe(II)/Cu(II) interaction. These findings improve our general understanding of the Fe(II)/Cu(II) interaction on soil iron redox chemistry for polychlorinated pesticide detoxification and heavy metal immobilization.

Dynamics of the microbial community and Fe(III)-reducing and dechlorinating microorganisms in response to pentachlorophenol transformation in paddy soil

Soil microorganisms play crucial roles in the fates of pollutants, and understanding the behaviour of these microorganisms is critical for the bioremediation of PCP-contaminated soil. However, shifts remain unclear in the community structure and Fe(III)-reducing and dechlorinating microorganisms during PCP transformation processes, especially during the stages from the lag to the dechlorination phase and from the dechlorination to the stationary phase. Here, a set of lab-scale experiments was performed to investigate the microbial community dynamics accompanying PCP transformation in paddy soil. 19μM of PCP was biotransformed completely in 10days for all treatments. T-RFLP analysis of the microbial community confirmed that Veillonellaceae and Clostridium sensu stricto were the dominant groups during PCP transformation, and the structures of the microbial communities changed due to the degree of biotransformation and the addition of lactate and AQDS. However, similar temporal dynamics of the microbial communities were obtained among all treatments. Furthermore, as revealed by quantitative PCR, the dynamics of Fe(III)-reducing and dechlorinating microorganisms, including Geobacter sp., Shewanella sp., and Dehalobacter sp., were consistent with the transformation kinetics of PCP, suggesting the critical roles played by these microorganisms in PCP transformation. These findings are valuable for making predictions of and proposing methods for the microbial detoxification of residual organochlorine pesticides in paddy soil.

The effect of ammonium chloride and urea application on soil bacterial communities closely related to the reductive transformation of pentachlorophenol

Pentachlorophenol (PCP) is widely distributed in the soil, and nitrogen fertilizer is extensively used in agricultural production. However, studies on the fate of organic contaminants as affected by nitrogen fertilizer application have been rare and superficial. The present study aimed to examine the effect of ammonium chloride (NH4Cl) and urea (CO(NH2)2) application on the reductive transformation of PCP in a paddy soil. The study showed that the addition of low concentrations of NH4Cl/CO(NH2)2 enhanced the transformation of PCP, while the addition of high concentrations of NH4Cl/CO(NH2)2 had the opposite effect. The variations in the abundance of soil microbes in response to NH4Cl/CO(NH2)2 addition showed that both NH4Cl and CO(NH2)2 had inhibitory effects on the growth of dissimilatory iron-reducing bacteria (DIRB) of the genus Comamonas. In contrast, for the genus Shewanella, low concentrations of NH4Cl inhibited growth, and high concentrations of NH4Cl enhanced growth, whereas all concentrations of CO(NH2)2 showed enhancement effects. In addition, consistent patterns of variation were found between the abundances of dechlorinating bacteria in the genus Dehalobacter and PCP transformation rates under NH4Cl/CO(NH2)2 addition. In conclusion, nitrogen application produced variations in the structure of the soil microbial community, especially in the abundance of dissimilatory iron-reducing bacteria and dechlorinating bacteria, which, in turn, affected PCP dechlorination.

Iron reduction coupled to reductive dechlorination in red soil: A review

Abstract Iron reduction plays an important role in the reductive transformation of organochlorine pesticides in red paddy soils. This interaction between iron reduction and organochlorine pesticides (OCPs) in red soils in south China is particularly important because of the high abundance and reactivity of iron within unique man-made paddy ecosystems. However, the relationships between iron reduction and reductive dechlorination and the geochemical constraints of these relationships are not fully understood. In this comprehensive review, we summarized current understanding of iron reduction, reductive dechlorination, the relationships between them, and their interactions with the reduction of nitrate, sulfate, Cu(II), and humic substances in red soils. Recent studies showed that iron reduction and dechlorination occur simultaneously in soils and that iron reduction could either stimulate or inhibit dechlorination. Meanwhile, sulfate and Cu(II) reduction can stimulate or inhibit iron reduction and dechlorination. Nitrate reduction can be coupled to iron reduction, but it inhibits dechlorination. Increasing evidence showed that humic substances can enhance the rates of both iron reduction and dechlorination by accelerating electron transfer. However, there is insufficient information in the literature for delineating the effects of several rising environmental problems (e.g., heavy metal pollution, deficiencies in phosphorus and aluminum) on iron reduction, OCP transformation, and the related microbial activities. Future studies are necessary because such information may be key for sustainable development of agriculture and pollution control in red soils.

Fractionation characteristics of rare earth elements (REEs) linked with secondary Fe, Mn, and Al minerals in soils

Soil secondary minerals are important scavengers of rare earth elements (REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs (La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs (LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs (HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs. The substantial fractions of REEs in soils extracted by oxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile (10xa0%–30xa0%), which were similar to the mass fraction of Fe (10xa0%–20xa0%). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation.

Thallium in flowering cabbage and lettuce: Potential health risks for local residents of the Pearl River Delta, South China

Thallium (Tl), a rare metal, is universally present in the environment with high toxicity and accumulation. Thalliums behavior and fate require further study, especially in the Pearl River Delta (PRD), where severe Tl pollution incidents have occurred. One hundred two pairs of soil and flowering cabbage samples and 91 pairs of soil and lettuce samples were collected from typical farmland protection areas and vegetable bases across the PRD, South China. The contamination levels and spatial distributions of soil and vegetable (flowering cabbages and lettuces) Tl across the PRD were investigated. The relative contributions of soil properties to the bioavailability of Tl in vegetables were evaluated using random forest. Random forest is an accurate learning algorithm and is superior to conventional and correlation-based regression analyses. In addition, the health risks posed by Tl exposure via vegetable intake for residents of the PRD were assessed. The results indicated that rapidly available potassium (K) and total K in soil were the most important factors affecting Tl bioavailability, and the competitive effect of rapidly available K on vegetable Tl uptake was confirmed in this field study. Soil weathering also contributed substantially to Tl accumulation in the vegetables. In contrast, organic matter might not be a major factor affecting the mobility of Tl in most of the lettuce soils. Fe and manganese (Mn) oxides also contributed little to the bioavailability of Tl. A risk assessment suggested that the health risks for Tl exposure through flowering cabbage or lettuce intake were minimal.