As common heavy metal pollutants, chromium (Cr) and arsenic (As) mainly come from the industrial waste residues and wastewater from mining and smelting industries. Industrial wastewater containing Cr and As may also be transferred into solid wastes after treatment. Cr waste residue (COPR) derived from production and use of chromate consists of many complex solid pha ses. The highly toxic hexavalent chromium (Cr (VI)) often forms solid solution with components and is encapsulated in the lattice of the phase of residue. In addition, the large amount of arsenic sulfide residue, produced in the treatment of acid sewage by sulfurization method, brings great challenges to storage and treatment because of its small particles and high water content. Therefore, t he highly mobile Cr (VI) and highly toxic As (III) from waste residues without appropriate treatment cause serious ha rm to water environment and soil, and finally threaten human health. In this paper, COPR and arsenic sulfide residue were treated by hydrothermal method respectively to realize the non toxic discharge of residue and resources recovery, and the mechanisms o f
the phase transformation in COPR and the phase transformation and fixation in arsenic sulfide residue were explore respectively in the proc ess of hydrothermal treatment. On this basis, low concentration heavy metal wastewater from waste residue treatment needs to be further treated. Taking the composite wastewater containing Cr(VI) and As(III) as the example, the photocatalytic technology by ferric alginate (Fe SA) as photocatalyst was used to convert Cr(VI) and As(III) into Cr(III) and As(V) with lower t oxicity, which are easy to precipitate and adsorb, and can realize the u ltimate removal of Cr and As.
The main findings and conclusions are as follows:
(1) Extraction of Cr(VI) from COPR by hydrothermal treatment assisted with sodium silicate (HT Na 2 SiO 3
In order to achieve highly efficient extraction of Cr(VI), HT Na 2 SiO 3 method was used to detoxify COPR, and the results reveal that Na 2 SiO 3 greatly promotes the extraction of Cr(VI) in COPR. After the treatment, the Cr(VI) concentration in the leachate by toxicity characteristic leaching procedure (TCLP) markedly decreased to 1.1 mg L 1 , well below the regulatory limit of 1.5 mg L 1 (GB 16889 2008, China EPA). X ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) mapping confirm t hat neither Cr(VI) nor Cr(III) were detected on the surface of COPR after treatment. The main mechanism of extracting Cr(VI) from COPR by HT Na 2 SiO 3 method was obtained, through the analysis of phase composition in COPR and further the results of the solid phase simulation experiments. In the hydrothermal conditions, SiO 32 ions not only act as the anion exchange agent to delaminate layered phases in COPR like hydrotalcites to release the encapsulated Cr(VI), but also as an alkaline reagent to help the oxidation of Cr(III) in COPR to soluble Cr(VI) both of which distinctively increased the leaching of chromium from COPR. This study provides a novel st rategy for both COPR detoxification and recovery of Cr(VI) for resource recycling.
(2) Conversion and stabilization of arsenic sulfide residue by hydrothermal treatment assisted with ferric nitrate (HT-Fe(NO3)3)By HT-Fe(NO3)3 method, arsenic sulfide resid ue can be transformed and stabilized in one step to form stable scorodite and recover sulfur. The results show that under the conditions of pH 1.0, Fe(NO3)3 dosage of 10 mmol g 1 (arsenic sulfide residue) and 150 o C , arsenic sulfide is dissolved, oxidized and stabilized by HT Fe(NO3)3 for 20 h. The arsenic capturing rate reaches over 99% because of the formation of the scorodite with a ~15 µm particle size. During the process of hydro thermal growth , the amorphous iron arsenate was formed in the early stage . It is possible that due to the dissolution and oxidation of sulfur in arsenic sulfide,amorphous iron arsenate nanoparticles would not grow further after crystallizing into lamellar structure, but form mesocrystals through the directional assembly proces s of lamellar structure and then grow into spherical large particles with polyhedral surface. S corodite with large particle size has a good stability and t he As concentration in the leachate from scorodite by TCLP is well below the regulatory limit and the produced elemental sulfur can be recycled. This study pr ovides a
simple and efficient strategy for the stabilization of arsenic sulfide residue.
(3) Study on the mechanism of photocatalytic redox and removal of Cr(VI)/As(III)wastewater by Fe SA Multiple heavy metal ions (e.g. Cr(VI) and As(III)) normally co exist in acid wastewater, making the wastewater treatment complicated. Herein, the synergistic redox conversion and removal of Cr(VI)/As(III) were effectively achieved by applying iron(III) cr oss linking alginate hydrogel beads (Fe SA) as photocatalyst under simulated sunlight. Results show that not only the complete redox conversion of 5 mg L 1 Cr(VI) and 5 mg L 1 As(III) was obtained in 150 min at pH 3.0 3.0, but also the removal efficiency of th e transformed products (Cr(III) and As(V)) was greatly enhanced to above 80% in a wide pH range of 3 7. The ∙CO 2 rradicals, produced by the ligand to metal charge transfer (LMCT) excitations on Fe-SA, together with the phtocatalysis generated Fe(II), was r esponsible for the Cr(VI) reduction. Meanwhile, a series of free radical reactions occur to form reactive oxygen species with the participation of O 2 and Fe (II) and CO 2 rradicals. Finally, OH mainly contributed to the As(III) oxidationoxidation, as confirmed by electron paramagnetic resonance (EPREPR) and the by-product of CO 22. Moreover, Fe-SA composite presented excellent reusability and performance in treatment of Cr(VI)/As(III) in real waters.