Variation on leaching behavior of caustic compounds in bauxite residue during dealkalization process.

Bauxite residue, a byproduct of alumina manufacture, is a serious environmental pollutant due to its high leaching contents of metals and caustic compounds. Four typical anions of CO32-, HCO3-, Al(OH)4- and OH- (represented caustic compounds) and metal ions (As, B, Mo and V) were selected to assess their leaching behavior under dealkalization process with different conditions including liquid/solid ratio (L/S ratio), temperature and leaching time. The results revealed that washing process could remove the soluble composition in bauxite residue effectively. The leaching concentrations of typical anions in bauxite residue decreased as follows: c(CO32-) > c(HCO3-) > c[Al(OH)4-] > c(OH-). L/S ratio had a more significant effect on leaching behavior of OH-, whilst the leaching concentration of Al(OH)4- varied larger underleaching temperature and time treatment. Under the optimal leaching, the total alkaline, soluble Na concentrations, exchangeable Ca concentrations were 79.52, 68.93, and 136.0 mmol/L, respectively, whilst the soluble and exchangeable content of As, B, Mo and V in bauxite residue changed slightly. However, it should be noted that water leaching has released metal ions such as As, B, Mo and V in bauxite residue to the surrounding environment. The semiquantitative analysis of XRD revealed that water leaching increased the content of gismondine from 2.4% to 6.4%. The SEM images demonstrated the dissolution of caustic compounds on bauxite residue surface. The correlation analysis indicated that CO32- and HCO3- could effectively reflect the alkalinity of bauxite residue, and may be regarded as critical dealkalization indicators to evaluate alkalinity removal in bauxite residue.

Advanced treatment of industrial wastewater by ozonation with iron-based monolithic catalyst packing: From mechanism to application.

An Fe-based catalyst was prepared by oxidising waste Fe shavings directly in a solution. In engineering applications, Fe shavings were compressed and modified to form Fe-based monolithic catalyst packing. Both of which exhibited excellent catalytic activity in catalytic ozonation industrial wastewater after biochemical treatment. Fe-based monolithic catalyst packing has irregular channels, large porosity, small pore diameter, and the effective specific surface area (SSA) up to 3500 m2/m3, these characteristics are conducive to mass transfer, and promote the effective utilisation of •OH in the catalyst "action zone". A tower reactor (<3000 m3/d) and reinforced concrete construction reactor (>5000 m3/d) were designed according to the wastewater flow. Regression analysis showed that hydraulic residence time (HRT) and O3/CODin are important parameters in engineering design and operation. In addition, strategies for the application of Fe-based monolithic catalyst packing to wastewater with high salinity and high inorganic carbon concentration have been proposed. Fe-based monolithic catalyst packing catalytic ozonation is a relatively cost-effective and eco-friendly process with extremely broad application prospects in the advanced treatment of industrial wastewater.

Effects of biochar/AQDS on As(III)-adsorbed ferrihydrite reduction and arsenic (As) and iron (Fe) transformation: Abiotic and biological conditions.

Microbe induced iron (Fe) reduction play an important role in arsenic (As) transformation and the related secondary mineral formation. Meanwhile biochar could react as electron shuttle for this process. Impact of biochar and model electron shuttle anthraquinone-2,6-disulfonate (AQDS) on the chemical/biological iron reduction of As(III)-adsorbed ferrihydrite and the solid-liquid redistribution of As in M1 buffer were studied. Fe reduction results in the release of As adsorbed on ferrihydrite into the solution. Under abiogenic conditions, both biochar and AQDS promoted ferrous production, the chemical oxidation of As(III) and As release. Inoculate with Shewanella oneidensis MR-1, AQDS has greater electronic shuttle function than biochar (with the maximum Fe(II) contents: 154 mg/L > 76.6 mg/L respectively). However, only 12.8 mg/L As was released in the presence of AQDS, which was much lower than that in the presence of biochar (21.6 mg/L), and may be associated with the transformation of As speciation and the formation of secondary minerals. XRD and EDX-SEM confirmed that the As could be fixed by the generated secondary mineral vivianite. The relative contents of vivianite in biological control and AQDS addition were 2.7% and 18.4%, respectively. This study provides information on the transformation and migration of As and Fe with the addition of biochar under anaerobic conditions, which is potential to understand the mechanism of As(III)-contaminated soil remediation.

The effects of biochar as the electron shuttle on the ferrihydrite reduction and related arsenic (As) fate.

The effects of electron shuttles (biochar/anthraquinone-2,6-disulphonate (AQDS)) on the process of the Shewanella oneidensis MR-1-induced As(V)-adsorbed ferrihydrite reduction were studied. The results showed that biochar could stimulate Fe(Ⅱ) and As release during the ferrihydrite bioreduction. After the addition of biochar, more dissolved organic matter (DOM) can be consumed as an electron donor to promote the metabolism of microorganisms by the fluorescence excitation-emission matrix spectra analysis. After microbial treatment, cyclic voltammetry (CV) showed that a unique cathodic peak and a distinct anodic peak appeared, which may represent the reduction of Fe(OH)3 to Fe(OH)2 and the complexed oxidation of Fe2+ to Fe3+. No characteristic peak was associated with arsenate reduction or arsenite oxidation. The mineralogical characterization of the final products indicated that AQDS can promote solid-state conversion from ferrihydrite to vivianite (Fe3(PO4)2·8H2O). However, the addition of biochar inhibited solid-state conversion of ferrihydrite. It was shown that after 6 d, the secondary mineral vivianite production in the bacteria alone and AQDS treatments was 8.12% and 15.6% respectively by mössbauer spectroscopy analysis. Moreover, the XPS indicated that As(V) has no species transformation. It provided new data for understanding the iron-reducing bacteria induced mineralization process and related biogeochemical cycles of Fe and As.

Effect of sulfur and sulfur-iron modified biochar on cadmium availability and transfer in the soil-rice system.

Cadmium (Cd) contamination in paddy soils has aroused global concern. Sulfur modified biochar (BC) could combine the benefits of BC and S for Cd remediation. However, no information is available on the impact of sulfur modified biochar on Cd phytoavailability in paddy soils. In this study, a pot experiment was conducted to investigate the effect of sulfur modified biochar (S-BC) and sulfur and iron (Fe) modified biochar (S-Fe BC) on Cd mobility and Cd transfer in the soil-rice system. The application of S-BC and S-Fe BC effectively reduced pore water Cd in the rhizosphere and non-rhizosphere pore water throughout the rice growth stages. S-BC and S-Fe BC addition increased the total chlorophyll content, as well as the root, shoot and grain biomasses of rice. Furthermore, S-BC and S-Fe BC amendments greatly increase the formation of Fe plaque on rice root surface, thus decreasing Cd accumulation in different rice tissues. In particular, S-Fe BC supplementation significantly reduced the Cd concentration in rice grains to 0.018 mg kg-1 in Cd-contaminated soil, which was lower than the China National standard for food contamination limit (0.2 mg kg-1 Cd). Sequential extraction results showed that S-BC and S-Fe BC can promote the transfer of exchangeable Cd to Fe-Mn oxide, organic and residual bound forms which reduce Cd in paddy soils. Thus, the amendment of S-Fe BC to Cd-contaminated paddy soil is an effective strategy to decrease Cd accumulation in rice grains and thereby protect public health.