[Research Progress on Solidification and MICP Remediation of Soils in Heavy Metal Contaminated Site].

Heavy metal pollution in soils of smelting sites is an important environmental problem to be solved urgently. Solidification technology has become one of the mainstream technologies for heavy metal remediation in contaminated sites owing to its shorter remediation time, low cost, and high treatment efficiency. On the basis of summarizing the latest research progress on the remediation of heavy metal pollution in sites by solidification in the past 10 years, this study focused on the mechanisms of solidification technology and analyzed the advantages and disadvantages of different mechanisms (mechanism of inorganic materials, mechanism of organic materials, mechanism of mechanical ball milling, and mechanism of microbial-induced carbonate mineralization (MICP)) and their scope of application. Then, according to the research focus and development trend presented by CiteSpace, the application prospects and limiting factors of MICP technology for the solidification and remediation of heavy metal pollution in sites were summarized from three aspects:the application of MICP in multi-metal remediation, the application of MICP composites in contaminated sites, and the influencing factors of MICP technology application. Finally, the prospects and challenges in solidification technology were put forward in order to provide reference for the future development.

Selective Electrocatalytic Water Oxidation to Produce H2O2 Using a C,N Codoped TiO2 Electrode in an Acidic Electrolyte.

Production of hydrogen peroxide (H2O2) via in situ electrochemical water oxidation possesses great potential applications in the energy and environment fields. In this work, for the first time, we reported a C,N codoped TiO2 electrode for selective electrocatalytic water oxidation to produce H2O2 in an acidic electrolyte. An electrochemical anodic oxidation method combined with postcalcination in the presence of urea was applied to fabricate such a C,N codoped TiO2 electrode, which was evidenced by detail structural characterizations. The calcination temperature and urea atmosphere were found to play key roles in its catalytic performances; the optimized 600N sample exhibited an onset potential of 2.66 V (vs Ag/AgCl) and a Tafel slope of 51 mV dec-1 at pH 3. Under the optimal applied potential, the cumulative H2O2 concentration for this sample reached 0.29 µmol L-1 cm-2 h-1. More importantly, a simple recalcination strategy was developed to recover the deactivation electrode. This study proposed an efficient C,N codoped TiO2 electrode toward water oxidation to selectively produce H2O2 in the acidic electrolyte, which could be further used to in situ generate H2O2 for the energy- and environment-related fields with water as the precursor.

[Risk Analysis of Heavy Metal Contamination in Farmland Soil Around a Bauxite Residue Disposal Area in Guangxi].

Bauxite residue disposal areas (BRDA) appear to result in the heavy metal pollution of the farm fields surrounding them. In total, 194 topsoil samples were collected from the fields surrounding a BRDA in Guangxi in order to comprehensively understand the pollutant characteristics. These characteristics and their ecological risks were assessed by the Nemerow and Harkanson indices, whilst the sources and correlations of eight heavy metals (V, Cr, Ni, Cu, Zn, As, Pb, and Co) were analyzed by means of the spatial interpolation method, correlation analysis, and principal component analysis (PCA). The results demonstrated that the surrounding fields were seriously polluted by heavy metals. Ninety-two percent of samples were polluted, including 36% that showed serious pollution, and As was the dominant contaminant. The ecological risk results showed that the risks of the surrounding fields were medium, and As was responsible for 68% of this. Spatial interpolation suggested that concentrations of heavy metals in the northeastward and southwestward areas were higher, however the southeastward areas were lower. Multivariate statistics indicated that the possible source of As contaminant was different to those of V, Ni, Zn, Pb, and Co; As was primarily influenced by anthropogenic contamination, including atmospheric sedimentation, and agricultural fertilization. Cr was affected by both soil parent material and atmospheric sedimentation, whereas V, Ni, Zn, Pb, And Co levels were mainly affected by soil parent material.

Arsenic sorption by red mud-modified biochar produced from rice straw.

Red mud-modified biochar (RM-BC) has been produced to be utilized as a novel adsorbent to remove As because it can effectively combine the beneficial features of red mud (rich metal oxide composition and porous structure) and biochar (large surface area and porous structure properties). SEM-EDS and XRD analyses demonstrated that red mud had loaded successfully on the surface of biochar. With the increasing of pH in solution, arsenate (As(V)) adsorption on RM-BC decreased while arsenite (As(III)) increased. Arsenate adsorption kinetics process on RM-BC fitted the pseudo-second-order model, while that of As(III) favored the Elovich model. All sorption isotherms produced superior fits with the Langmuir model. RM-BC exhibited improved As removal capabilities, with a maximum adsorption capacity (Qmax) for As(V) of 5923 µg g-1, approximately ten times greater than that of the untreated BC (552.0 µg g-1). Furthermore, it has been indicated that the adsorption of As(V) on RM-BC may be strongly associated with iron oxides (hematite and magnetite) and aluminum oxides (gibbsite) by X-ray absorption near-edge spectroscopy (XANES), which was possibly because of surface complexation and electrostatic interactions. RM-BC may be used as a valuable adsorbent for removing As in the environment due to the waste materials being relatively abundant.

Oxic and anoxic conditions affect arsenic (As) accumulation and arsenite transporter expression in rice.

Arsenic (As) exposure from rice consumption has now become a global health issue. This study aimed to investigate the effects of rice rhizosphere oxic conditions on silicate transporter (responsible for arsenite transportation) expressions, and on As accumulation and speciation in four rice genotypes, including two hybrid genotypes (Xiangfengyou9, Shenyou9586) and two indica subspecies (Xiangwanxian17, Xiangwanxian12). Oxic and anoxic treatments have different effects on root length (p < 0.001) and weight (p < 0.05). Total As concentrations in roots were dramatically lower in oxic treatments (88.8-218 mg/kg), compared to anoxic treatments (147-243 mg/kg) (p < 0.001). Moreover, root and shoot arsenite concentrations in oxic treatments were lower than that in anoxic treatments in arsenite treatments. The relative abundance of silicate transporter expressions displayed a trend of down-regulation in oxic treatments compared to anoxic treatments, especially significantly different for Xiangwanxian17, Xiangwanxian12 in Lsi1 expressions (p < 0.05), Xiangfengyou9, Shenyou9586, Xiangwanxian17 in Lsi2 expressions (p < 0.05). However, there were no significant differences of transporter expressions in different As treatments and genotypes. It may be a possible reason for low As accumulation in rice growing aerobically compared to flooded condition and a potential route to reduce the health risk of As in rice.

An investigation of cellular distribution of manganese in hyperaccumlator plant Phytolacca acinosa Roxb. using SRXRF analysis.

Phytolacca acinosa Roxb. (P. acinosa) is a recently discovered manganese hyperaccumulator plant from southern China. It is a good candidate for phytoremediation of manganese(Mn) polluted soil for its high biomass and fast growth. Knowledge of the tissue localization and identification of heavy metals can provide essential information on metal toxicity and bioaccumulation mechanisms. Synchrotron radiation X-ray fluorescence spectroscopy (SRXRF) microprobe was used in this study to investigate the cellular distributions of Mn and other elements in root, stem, leaf, petiole and midrib of P. acinosa. The highest Mn content was found in the vascular tissues of root, stem, petiole and midrib. Cortex in root played a key role in Mn absorption and Mn was limited in the vascular bundle during the process of transportation in stem. Moreover, Mn content in leaf epidermis was higher than that in mesophyll, which suggested that the sequestration of Mn in leaf epidermis might be one of the detoxification mechanisms of P. acinosa. The significance of other elemental (such as P, S, K, Ca, Fe, Zn and Cu) distribution patterns and the correlation with Mn were also discussed.

Effect of silicate on arsenic fractionation in soils and its accumulation in rice plants.

Four rice genotypes, two hybrid and two indica, were selected to investigate the effects of silicate (Si) application on arsenic (As) accumulation and speciation in rice and As fractionation in soil. There were significant differences in root, straw and grain biomass among genotypes (p < 0.05), and Si application significantly increased root (p < 0.05) and grain biomass (p < 0.001). Silicate addition reduced the proportion of As associated with well-crystallized hydrous oxides of Fe and Al and residual phases, whilst increasing the proportions of specifically-sorbed As and As associated with amorphous and poorly-crystalline Fe and Al hydrous oxides. Furthermore, the results indicated that the fraction proportions of non-specifically sorbed, specifically-sorbed, and associated with amorphous and poorly-crystalline hydrous oxides of Fe and Al in rhizosphere soils, were greater than non-rhizosphere soils. Silicate application had a significant effect decreasing total As concentrations in root (p < 0.005), straw (p < 0.05) and husk (p < 0.001) of rice plants. The effect of Si on reducing As accumulation in rice leaves was revealed by SXRF. Indica genotypes transported and accumulated less As than hybrid genotypes. Both percentage and concentration of iAs were lower in indica genotype XFY-9 than in hybrid genotype XWX-12. Silicate reduced iAs and DMA by 21% and 58% in grain (polished) respectively. DMA may have a greater translocation capacity from straw to grain (polished) than inorganic As. The study provides the potential for understanding As uptake mechanisms in rice and mitigating the health risks posed by As contamination in paddy fields.

The effect of silicon on iron plaque formation and arsenic accumulation in rice genotypes with different radial oxygen loss (ROL).

Rice is one of the major pathways of arsenic (As) exposure in human food chain, threatening over half of the global population. Greenhouse pot experiments were conducted to examine the effects of Si application on iron (Fe) plaque formation, As uptake and rice grain As speciation in indica and hybrid rice genotypes with different radial oxygen loss (ROL) ability. The results demonstrated that Si significantly increased root and grain biomass. Indica genotypes with higher ROL induced greater Fe plaque formation, compared to hybrid genotypes and sequestered more As in Fe plaque. Silicon applications significantly increased Fe concentrations in iron plaque of different genotypes, but it decreased As concentrations in the roots, straws and husks by 28-35%, 15-35% and 32-57% respectively. In addition, it significantly reduced DMA accumulation in rice grains but not inorganic As accumulation. Rice of indica genotypes with higher ROL accumulated lower concentrations of inorganic As in grains than hybrid genotypes with lower ROL.

Uptake and accumulation of phenanthrene and pyrene in spiked soils by Ryegrass (Lolium perenne L.).

Phytoremediation has long been recognized as a cost-effective method for the removal of polycyclic aromatic hydrocarbons (PAHs) from soil. A study was conducted to investigate the uptake and accumulation of PAHs in root and shoot of Lolium perenne L. Pot experiments were conducted with series of concentrations of 3.31-378.37 mg/kg for phenanthrene and those of 4.22-365.38 mg/kg for pyrene in a greenhouse. The results showed that both ryegrass roots and shoots did take up PAHs from spiked soils, and generally increased with increasing concentrations of PAH in soil. Bioconcentration factors (BCFs) of phenanthrene by shoots and roots were 0.24--4.25 and 0.17-2.12 for the same treatment. BCFs of pyrene by shoots were 0.20--1.5, except for 4.06 in 4.32 mg/kg treatment, much lower than BCFs of pyrene by roots (0.58--2.28). BCFs of phenanthrene and pyrene tended to decrease with increasing concentrations of phenanthrene and pyrene in soil. Direct uptake and accumulation of these compounds by Lolium perenne L. was very low compared with the other loss pathways, which meant that plant-promoted microbial biodegradation might be the main contribution to plant-enhanced removal of phenanthrene and pyrene in soil. However, the presence of Lolium perenne L. significantly enhanced the removal of phenanthrene and pyrene in spiked soil. At the end of 60 d experiment, the extractable concentrations of phenanthrene and pyrene were lower in planted soil than in non-planted soil, about 83.24%--91.98% of phenanthrene and 68.53%-84.10% of pyrene were removed from soils, respectively. The results indicated that the removal of PAHs in contaminated soils was a feasible approach by using Lolium perenne L.