(Im)mobilization and speciation of lead under dynamic redox conditions in a contaminated soil amended with pine sawdust biochar.

Biochar can reduce the mobility and availability of potentially toxic elements (PTEs) in soils and improve soil properties. However, immobilization efficiencies of biochar can be varied according to environmental conditions, such as pH and redox potential (Eh), especially for soils under flood-dry cycles. In the current study, biochar produced at 300 and 550 °C (referred as BC300 and BC550, respectively) and its feedstock (pine sawdust biomass, BM) were used to amend a lead (Pb)-contaminated soil under pre-defined redox windows (from -300 to +250 mV). Key features of the soil-solution were evaluated in detail, including pH, dissolved organic carbon, sulphate, and dissolved Al, Fe, and Mn. The BC550 reduced the amount of dissolved Pb and showed a different pattern of Eh-pH in the soil slurry compared with BM and BC300. This might be attributed to its higher alkalinity and surface area. The highest amount of dissolved Pb was found at slightly anoxic conditions (-100 to 0 mV) in CS (control soil), S&BM (soil amended with BM), and S&BC300 (soil amended with BC300), which could be associated with the dissolution of Fe/Mn oxides. Moreover, the fitting results of Pb X-ray absorption fine structure (XAFS) indicated that the proportion of Pb(CH3COO)2 was decreasing when changing from anoxic to oxic condition in S&BC300, while the Pb speciation pattern in soil was stable in S&BC550. These results suggested that BC550 is more suitable amendment for Pb immobilization than BM and BC300 in this study. In addition, biochar produced at higher temperatures can be more stable so it can be suitable for remediation of Pb-contaminated soils which are frequently flooded.

Mobilization of mercury species under dynamic laboratory redox conditions in a contaminated floodplain soil as affected by biochar and sugar beet factory lime.

Mercury and its species are toxic and therefore strategies to immobilize them or to impede the formation of bioaccumulative MeHg are a hot topic of ongoing research. Biochar (BC) and sugar beet factory lime (SBFL) are suggested to have the potential to meet these goals. However, their ability to restrain the mobilization of total Hg (Hgt), methylmercury (MeHg), and ethylmercury (EtHg) or the formation of MeHg and EtHg has not been examined to date. Moreover, the effect of systematically altered redox conditions on the release dynamics of Hgt, MeHg, and EtHg in a contaminated floodplain soil as affected by these soil amendments has not been studied. Therefore, we investigated the impact of pre-defined redox conditions on the release dynamics of Hgt, MeHg, and EtHg in a contaminated floodplain soil (CS) and the soil amended with either BC (CS+BC) or SBFL (CS+SBFL). The mobilization of Hgt, MeHg, and EtHg was generally higher at low redox potential (EH) and decreased with increasing EH, irrespective of soil treatment. Both BC and SBFL diminished the release of Hgt from soil but not the methylation and ethylation of Hg. In CS+SBFL approximately half of Hgt was found in solution compared to CS. However, higher methylation efficiency (MeHg/Hgt ratio) was found in CS+SBFL counterbalancing this benefit. Abundances of specific phospholipid fatty acids suggest the presence of sulfate-reducing bacteria, which are considered as primary Hg methylators. The results indicate that both BC and SBFL have the potential to curtail the release of Hgt from inundated soils, while SBFL was more efficient. However, these amendments had no marked effect on the MeHg and EtHg concentrations. Therefore, further research should be conducted to identify soil additives that are capable to reduce the release and formation of these Hg species.

Impact of biochar on mobilization, methylation, and ethylation of mercury under dynamic redox conditions in a contaminated floodplain soil.

Mercury (Hg) is a highly toxic element, which is frequently enriched in flooded soils due to its anthropogenic release. The mobilization of Hg and its species is of ultimate importance since it controls the transfer into the groundwater and plants and finally ends in the food chain, which has large implications on human health. Therefore, the remediation of those contaminated sites is an urgent need to protect humans and the environment. Often, the stabilization of Hg using amendments is a reliable option and biochar is considered a candidate to fulfill this purpose. We tested two different pine cone biochars pyrolyzed at 200 °C or 500 °C, respectively, with a view to decrease the mobilization of total Hg (Hgt), methylmercury (MeHg), and ethylmercury (EtHg) and/or the formation of MeHg and EtHg in a contaminated floodplain soil (Hgt: 41 mg/kg). We used a highly sophisticated automated biogeochemical microcosm setup to systematically alter the redox conditions from ~-150 to 300 mV. We continuously monitored the redox potential (EH) along with pH and determined dissolved organic carbon (DOC), SUVA254, chloride (Cl-), sulfate (SO42-), iron (Fe), and manganese (Mn) to be able to explain the mobilization of Hg and its species. However, the impact of biochar addition on Hg mobilization was limited. We did not observe a significant decrease of Hgt, MeHg, and EtHg concentrations after treating the soil with the different biochars, presumably because potential binding sites for Hg were occupied by other ions and/or blocked by biofilm. Solubilization of Hg bound to DOC upon flooding of the soils might have occurred which could be an indirect impact of EH on Hg mobilization. Nevertheless, Hgt, MeHg, and EtHg in the slurry fluctuated between 0.9 and 52.0 µg/l, 11.1 to 406.0 ng/l, and 2.3 to 20.8 ng/l, respectively, under dynamic redox conditions. Total Hg concentrations were inversely related to the EH; however, ethylation of Hg was favored at an EH around 0 mV while methylation was enhanced between -50 and 100 mV. Phospholipid fatty acid profiles suggest that sulfate-reducing bacteria may have been the principal methylators in our experiment. In future, various biochars should be tested to evaluate their potential in decreasing the mobilization of Hg and to impede the formation of MeHg and EtHg under dynamic redox conditions in frequently flooded soils.

Pine sawdust biomass and biochars at different pyrolysis temperatures change soil redox processes.

To date, no investigation has been carried out to explore the effects of biochars produced at different pyrolysis temperatures on the dynamics of redox potential (EH) and pH in a contaminated floodplain soil. Thus, we aimed to quantify the dynamics of EH and pH in contaminated flooded soils treated with 70tha-1 of pine sawdust biomass (S&BM) and biochars pyrolyzed at 300°C (S&BC300) and 550°C (S&BC550) and pre-incubated for 105days in an automated biogeochemical microcosm system. Microbial community composition was also determined via analyzing phospholipid fatty acid (PLFA).We found that BC300 and BC550 treatments substantially decreased (3-6.5%) and BM increased (~37%) the mean of soil EH compared to the untreated contaminated soil (CS).·The largest EH decline in S&BC550 was at the rate of -80mVh-1 at 10h while it was observed at 25h in S&BC300 and 20-25h in S&BM or CS, respectively. At high EH, a higher total PLFA biomass and microbial groups in the CS (71-87%) were found in comparison to treated soils. Higher aromaticity and ash content in BC550 than BC300 and BM led to the greater PLFA biomass and microbial groups which contributed to higher capacity of accepting and donating electrons in soil slurry and were probably one reason for the largest decrease in EH. Pine sawdust biomass and BCs have a noticeable influence in soil biogeochemical processes relevant to fluctuating redox conditions.

Mobility and phytoavailability of As and Pb in a contaminated soil using pine sawdust biochar under systematic change of redox conditions.

Biochar has been adopted to control the mobility and phytoavailability of trace elements (TEs) in soils. To date, no attempt has been made to determine the mobility and phytoavailability of arsenic (As) and lead (Pb) in a contaminated soil with biochars as amendments under predefined redox potentials (EH). Thus, in this study, a soil contaminated with As and Pb (2047 and 1677 mg kg-1, respectively) was pre-incubated for 105 days with three amendments (pine sawdust biomass (BM) and two biochars produced from the same feedstock at 300 °C (BC300) and 550 °C (BC550)). The aged samples were then exposed to dynamic EH conditions to evaluate the mobility and phytoavailability of As and Pb after immobilization. The BM amendment significantly decreased and the BC300 slightly reduced the mobility and phytoavailability of As and Pb, which may be related to the oxygen-containing functional groups on the surface of BM and BC300. In contrast, BC550 increased the mobility of As at -300 to -100 mV and 100 mV, enhanced the phytoavailability of As under oxidizing condition (>100 mV), but reduced the phytoavailability of Pb, which might be caused by the properties of amendments and redox chemistry of the TEs. The effectiveness of BM and biochars for the stabilization of As and Pb varied under dynamic EH conditions, which indicates that detailed investigations should be conducted before the applications of biochar as soil amendment under variable environmental conditions, especially for contaminated paddy soils.