Effect of dissimilatory iron and sulfate reduction on arsenic dynamics in the wetland rhizosphere and its bioaccumulation in wetland plants (Scirpus actus).

Microbial redox transformations of arsenic (As) are coupled to dissimilatory iron and sulfate reduction in the wetlands, however, the processes involved are complex and poorly defined. In this study, we investigated the effect of dissimilatory iron and sulfate reduction on As dynamics in the wetland rhizosphere and its bioaccumulation in plants using greenhouse mesocosms. Results show that high Fe (50µM ferrihydrite/g solid medium) and SO42- (5mM) treatments are most favorable for As sequestration in the presence of wetland plants (Scirpus actus), probably because root exudates facilitate the microbial reduction of Fe(III), SO42-, and As(V) to sequester As(III) by incorporation into iron sulfides and/or plant uptake. As retention in the solid medium and accumulation in plants were mainly controlled by SO42- rather than Fe levels. Compared to the low SO42- (0.1mM) treatment, high SO42- resulted in 2 times more As sequestered in the solid medium, 30 times more As in roots, and 49% less As in leaves. An As speciation analysis in pore water indicated that 19% more dissolved As was reduced under high SO42- than low SO42- levels, which is consistent with the fact that more dissimilatory arsenate-respiring bacteria were found under high SO42- levels.

Cadmium adsorption on plant- and manure-derived biochar and biochar-amended sandy soils: impact of bulk and surface properties.

To investigate the role of the bulk and surface composition of both biochar and biochar-amended soils in the adsorption of Cd(2+), as well as the influence of different biochars added to the soils on Cd(2+) adsorption, swine-manure-derived biochars (BSs) and wheat-straw-derived biochars (BWs) were produced at 300, 450, and 600°C. These biochars were added to a sandy soil to investigate the effect of biochars on the adsorption of Cd(2+) by soil. The significantly higher surface C content of the amended soils compared to their bulk C content suggests that the minerals of the biochar-amended soils are most likely covered primarily by biochars. The maximum adsorption capacity (Qmax,total) of the BSs was 10-15 times higher than that of the BWs due to the high polarity and ash content of the BSs. The polarity ((N+O)/C) of the low-temperature biochars greatly affected their Cd(2+) adsorption. The Qmax,total of the BS-amended soils increased with increasing dose, whereas the Qmax,total of the BW-amended soils showed the opposite behavior, which was attributed to the different surface composition characteristics of the two types of soil. The BSs were more effective in immobilizing Cd(2+) upon application to the soil relative to the BWs. This study elucidates the spatial distribution of biochars in biochar-amended soils and highlights the importance of the surface composition of the investigated samples in Cd(2+) adsorption.

Sorption of atrazine and phenanthrene by organic matter fractions in soil and sediment.

Atrazine and phenanthrene (Phen) sorption by nonhydrolyzable carbon (NHC), black carbon (BC), humic acid (HA) and whole sediment and soil samples was examined. Atrazine sorption isotherms were nearly linear. The single-point organic carbon (OC)-normalized distribution coefficients (K(OC)) of atrazine for the isolated HA1, NHC1 and BC1 from sediment 1 (ST1) were 36, 550, and 1470 times greater than that of ST1, respectively, indicating the importance of sediment organic matter, particularly the condensed fractions (NHC and BC). Similar sorption capacity of atrazine and Phen by NHC but different isotherm nonlinearity indicated different sorption domains due to their different structure and hydrophobicity. The positive relationship between (O+N)/C ratios of NHC and atrazine logK(OC) at low concentration suggests H-bonding interactions. This study shows that sediment is probably a less effective sorbent for atrazine than Phen, implying that atrazine applied in sediments or soils may be likely to leach into groundwater.

Sorption of endocrine disrupting chemicals by condensed organic matter in soils and sediments.

Sorption of 17alpha-ethinyl estradiol (EE2) and bisphenol A (BPA) by nonhydrolyzable carbon (NHC), black carbon (BC), and bulk soils and sediments was examined. All sorption isotherms were nonlinear and fitted both Freundlich and Dubinin-Ashtakhov (DA) models. The single-point organic carbon (OC)-normalized distribution coefficient (K(OC)) of EE2 for the isolated NHC and BC was 2.7-4.8 times and 5.4-12.9 times greater, respectively, than that of the bulk samples. However, no clear trend in BPA K(OC) values was observed. Based on the contribution of soil/sediment organic matter (SOM) fractions to the overall sorption of BPA or EE2 by the bulk samples, condensed SOM (NHC and BC) generally played a dominant role to the overall sorption. The BPA adsorption capacity (Q(OC)(0)) from the DA model was higher than that of EE2 on NHC and there was obvious difference in isotherm nonlinearity (n) between EE2 and BPA. These results suggest that BPA may have more access to the pore sites of NHC samples than EE2. The pi-pi bonds formed between BPA and NHC or BC may be stronger than that between EE2 and NHC or BC. This would be attributed to the fact that BPA has two benzene rings, and can also be used to explain the difference in hexadecane-water partition coefficient (K(HW))-normalized K(OC) values (K(OC)/K(HW)) of BPA and EE2 after factoring out the hydrophobic effect. These findings could be useful for predicting fate and ecological risks of endocrine disrupting chemicals (EDCs) (e.g., EE2 and BPA) in natural environments especially when soils or sediments become receptors for EDCs.

Organochlorine pesticides and polybrominated diphenyl ethers in irrigated soils of Beijing, China: levels, inventory and fate.

Limited information on the levels, inventory and fate of Organochlorine pesticides (OCPs) and Polybrominated diphenyl ethers (PBDEs) in the soils irrigated by sewage or wastewater is available. In this study, variation in concentrations, profiles and fate of OCPs and PBDEs were investigated using soil samples collected from a region irrigated by sewage, mixed water and clean water in the east of Beijing, China. No significant variation was observed among groups, except for penta-BDEs. The measured SigmaOCPs and SigmaPBDEs residues ranged from 6.4 to 171.2 ng g(-1) (dw) and 501.9 to 3310.7 pg g(-1) (dw), respectively. SigmaDDTs and BDE-209 were the most abundant congeners accounting for about 76% of SigmaOCPs and 93% of SigmaPBDEs. Concentrations of hexachlorocyclohexanes (HCHs), dichlorodiphenyltrichloroethane (DDTs) and its major degradation products, and hexachlorobenzene (HCB) ranged from 1.2 to 11.4 ng g(-1) (dw), 4.0 to 155.6 ng g(-1) (dw) and 0.3 to 3.4 ng g(-1) (dw), respectively. The major DDT degradation products were p,p'-DDT and p,p'-DDE. The major hexachlorocyclohexane (HCH) isomer in irrigated soils is beta-HCH, reflecting its higher affinity to solids and resistance to degradation than other isomers. Both alpha-HCH/beta-HCH and p,p'-DDT/p,p'-DDE ratios were log-normally distributed and negatively correlated to log(SigmaHCHs) and log(SigmaDDTs), respectively, suggesting no significant recent application of OCPs. Individual BDE congeners, SigmaPBDEs and SigmaOCPs were significantly correlated with total organic carbon (TOC). Moreover, a good correlationship between SigmaPBDEs and black carbon (BC) was obtained but not between SigmaOCPs and BC. Sewage irrigation did not have obvious effect on their contaminant levels and inventory of OCPs and PBDEs.