Effect of dissolved organic carbon from sludge, Rice straw and spent coffee ground biochar on the mobility of arsenic in soil.

To date, studies on the mobility of arsenic (As) in soil amended with biochar have primarily relied on broad empirical observations, resulting in a gap between the behavior of As in amended soil and the chemical mechanisms controlling that behavior. This study focuses on the influence of abiotic factors in As mobility in As-contaminated soils amended with biochar. In order to understand the leaching of DOC and phosphate across a range of biomass feedstock and pyrolysis temperature, rice straw and granular sludge from an anaerobic digester were pyrolyzed at 300, 550, and 700 °C, and subjected to leaching studies by mixing air dried soil with 10 wt% of biochar at a soil: water ratio of 1:1(w/v). The concentration of DOC in the presence of granular sludge biochar and rice straw biochar increased from 190 mg L-1 to 2605 mg L-1 and 1192 mg L-1, respectively, which considerable accelerated the mobilization of Fe and As. More specifically, DOC drove the reduction of Fe(III) to Fe(II). Our results suggest enhanced release of As via the reductive dissolution of iron oxides, including by the chelating-enhanced dissolution of Fe oxides, and competitive desorption by DOC and phosphate from biochar. The influence of DOC and phosphate was further evaluated using realistic application amounts (1, 3, and 5 wt%) of biochars derived from pyrolysis of granular sludge, rice straw and spent coffee ground at 300 and 550 °C. The results from these experiments further confirm that DOC is a key factor for influencing the mobility of As in the amendment of biochar to As-contaminated soil, which indicates that biochar having low levels of leachable carbon should be amended to As-contaminated soils, and with caution.

Simultaneous application of oxalic acid and dithionite for enhanced extraction of arsenic bound to amorphous and crystalline iron oxides.

To extract As bound to amorphous and crystalline iron oxides, this study proposed simultaneous application of oxalic acid and dithionite, which was observed to induce synergistic effect and accomplish effective extraction of As bound to both iron oxides. However, the formation of arsenic sulfide decreased overall removal of As because the insoluble precipitate form of As remained as a residual fraction of As in soil. Therefore, stepwise addition of dithionite in the simultaneous application was applied to minimize the formation of secondary minerals and maximize the As extraction. As a result, 74% of As bound to amorphous iron oxides and 65% of As bound to crystalline iron oxides were removed. More importantly, the stepwise application of oxalic acid and dithionite was effective to reduce the bioaccessible concentration of As in the treated soil. Therefore, the proposed application could reduce the potential risk of contaminated soil to human health by extraction-based remedial action.

Role of reducing agent in extraction of arsenic and heavy metals from soils by use of EDTA.

Although many metal-contaminated sites contain both anionic arsenic and cationic heavy metals, the current remediation technologies are not effective for the simultaneous removal of both anionic and cationic elements from the contaminated sites due to their different characteristics. In this study, the role of reducing agent in simultaneous extraction of As, Cu, Pb, and Zn from contaminated soils was investigated using EDTA. The addition of reducing agents, which includes sodium oxalate (Na2C2O4), ascorbic acid (C6H8O6) and sodium dithionite (Na2S2O4), greatly enhanced the EDTA extraction of both As and heavy metals from the contaminated soils due to the increased mobility of the metals under the reduced conditions. The extent of the enhancement of the EDTA extraction was greatly affected by the reducing conditions. Strong reducing conditions (0.1 M of dithionite) were required for the extraction of metals strongly bound to the soil, while weak reducing conditions (0.01 M of dithionite or 0.1 M of oxalate/ascorbic acid) were sufficient for extraction of metals that were relatively weakly bound to the soil. An almost 90% extraction efficiency of total metals (As, Cu, Zn, and Pb) was obtained from the contaminated soils using the combination of dithionite and EDTA. Our results clearly showed that the combination of dithionite and EDTA can effectively extract As and heavy metals simultaneously from soils under a wide range of pH conditions.

In situ field application of electrokinetic remediation for an As-, Cu-, and Pb-contaminated rice paddy site using parallel electrode configuration.

The applicability of an in situ electrokinetic process with a parallel electrode configuration was evaluated to treat an As-, Cu-, and Pb-contaminated paddy rice field in full scale (width, 17 m; length, 12.2 m; depth, 1.6 m). A constant voltage of 100 V was supplied and electrodes were spaced 2 m apart. Most As, Cu, and Pb were bound to Fe oxide and the major clay minerals in the test site were kaolinite and muscovite. The electrokinetic system removed 48.7, 48.9, and 54.5 % of As, Cu, and Pb, respectively, from the soil during 24 weeks. The removal of metals in the first layer (0-0.4 m) was higher than that in the other three layers because it was not influenced by groundwater fluctuation. Fractionation analysis showed that As and Pb bound to amorphous Fe and Al oxides decreased mainly, and energy consumption was 1.2 kWh/m(3). The standard deviation of metal concentration in the soil was much higher compared to the hexagonal electrode configuration because of a smaller electrical active area; however, the electrode configuration removed similar amounts of metals compared to the hexagonal system. From these results, it was concluded that the electrokinetic process could be effective at remediating As-, Cu-, and Pb-contaminated paddy rice field in situ.

Enhanced-electrokinetic extraction of heavy metals from dredged harbor sediment.

In this study, the feasibility of an ex situ electrokinetic (EK) process combined with pre-oxidation using hydrogen peroxide (H2O2) and pre-washing using ethylenediaminetetraacetic acid (EDTA) was investigated in enhancing the extraction of Cu, Pb, and Zn from actual dredged harbor sediment. H2O2 pre-oxidation led to a change in the fractionation of Cu bound to organic matter and the sulfide fraction in the Fe-Mn oxides to the exchangeable fraction, but was not effective at removing metals. In contrast, EDTA pre-washing changed the Fe-Mn oxide-bound fractions of Cu and Pb into easily extractable fractions; 20.1, 27.5, and 32.8% of Cu, Pb, and Zn were removed, respectively. During EK treatment, metals were transported toward the anode by electromigration of negatively charged complexes such as metal-EDTA and metal-citrate. However, EK treatment did not significantly enhance the removal of metals because metals accumulated near the anodic region with an increase in the exchangeable fraction due to the short EK operating duration and low voltage gradient. Therefore, it is necessary to extend the EK operating duration and/or increase the voltage gradient for effective transportation and removal of metals from sediment.

In situ electrokinetic remediation of As-, Cu-, and Pb-contaminated paddy soil using hexagonal electrode configuration: a full scale study.

We investigated the in situ applicability of the electrokinetic process with a hexagonal electrode configuration in order to remediate arsenic (As)-, copper (Cu)-, and lead (Pb)-contaminated paddy rice field soil at a field scale (width 17 m, length 12.2 m, and depth 1.6 m). An iron electrode was used in order to prevent the severe acidification of the soil near the anode. We selected ethylenediaminetetraacetic acid (EDTA) as a pursing electrolyte to enhance the extraction of Cu and Pb. The system removed 44.4% of the As, 40.3% of the Cu, and 46.6% of the Pb after 24 weeks of operation. Fractionation analysis showed that the As bound to amorphous ion (Fe) and aluminum (Al) oxyhydroxides was changed into a form of As specifically bound. In the case of Cu and Pb, the fraction bound to Fe-Mn oxyhydroxide primarily decreased. The EDTA formed negatively charged complexes with Cu and Pb, and those complexes were transported toward the anode. The energy consumption was very low compared to that on a small scale because there was less energy consumption due to Joule heating. These results show that the in situ electrokinetic process could be applied in order to remediate paddy rice fields contaminated with multiple metals.

Field application of electrokinetic remediation for multi-metal contaminated paddy soil using two-dimensional electrode configuration.

In this study, we evaluated the feasibility of in situ electrokinetic remediation for arsenic (As)-, copper (Cu)-, and lead (Pb)-contaminated soil, in a pilot-scale field application with two-dimensional electrode configurations. Square and hexagonal configurations with different electrode spacing, 1 m and 2 m, were investigated under a constant 100 V. A square configuration with electrode spacing of 2 m removed 61.5 % of As, 11.4 % of Cu, and 0.9 % of Pb, respectively, and a hexagonal configuration with the same spacing showed a higher removal efficiency in top (59 % of As, 0-0.5 m) and middle (53 % of As, 0.5-1.0 m) layers, but much lower removal efficiency in the bottom layer (1-1.5 m), which was thought to be due to groundwater flow through periodic rise and fall of tides. Fractionation analysis showed that As bound to Fe-Mn oxyhydroxide was the main form of As removed by the electrokinetic process. The two-dimensional configuration wasted less electrical energy by Joule heating, and required fewer electrode installations, compared to the one-dimensional electrode configuration.