Oxidation and reduction reactions of (Al/FexCr1-x)2O3 caused by CaO during thermal treatment of solid waste containing Cr.

Solid solutions of (AlxCr1-x)2O3 and (FexCr1-x)2O3 are predominant compounds containing Cr in solid waste and are frequently formed during thermal treatment of solid waste. (AlxCr1-x)2O3 and (FexCr1-x)2O3 have superior thermomechanical properties and excellent corrosion resistance. However, oxidation and reduction reactions of the Cr in these solid solutions seriously affect their chemical stabilities and the environmental risks posed by the final products. In this study, first the reaction behaviors of (AlxCr1-x)2O3 and (FexCr1-x)2O3 at high temperatures were analyzed and whether the incorporation of Cr(III) in solid solutions can prevent Cr(III) from being oxidized was determined. Both (AlxCr1-x)2O3 and (FexCr1-x)2O3 without the presence of CaO exhibit good thermal stability at high temperatures. However, the participation of CaO induces Cr(III) oxidation in (AlxCr1-x)2O3 and (FexCr1-x)2O3 at 500-1000 °C. Cr(III) oxidation in these solid solutions is accompanied by the formation of CaCrO4 and Fe2O3 or Al2O3. Al2O3 combines with CaCrO4 and further forms a more stable Cr(VI) compound (e.g., Ca4Al6O12CrO4). While Fe2O3 combines with CaCrO4 at 1000-1200 °C. This is accompanied by the formation of CaCr2O4 and CaFe2O4, which effectively promotes the reduction of Cr(VI). Moreover, part of the CaCr2O4 transforms into a more stable phase (i.e., FeCr2O4) at 1200-1300 °C. Although the incorporation of Cr(III) in these solid solutions cannot prevent Cr(III) oxidation completely at high temperatures, the Cr(III) oxidation in these solid solutions is still suppressed compared with Cr2O3. The results of this study provide further insights into the oxidation and reduction reactions of Cr-hosting compounds during thermal treatment of solid waste.

Self-floating capsule of algicidal bacteria Bacillus sp. HL and its performance in the dissolution of Microcystis aeruginosa.

Algicidal bacteria is considered as an efficient and environmentally friendly approach to suppress Microcystis aeruginosa (M. aeruginosa). However, algicidal bacteria in natural water is limited during the practical application due to the interference of external factors and the low reuse capability. In this study, a bio-degradation capsule for M. aeruginosa is prepared by bio-compatible sodium alginate (SA) compositing with eco-friendly ethyl cellulose (EC) to improve the property and reuse capability of algicidal bacteria. Bacterial strain HL was well immobilized and the capsule was obtained with 2% of SA, 3% of calcium chloride (CaCl2) and 3% of EC. It has been proved that capsules immobilizing bacteria HL shows considerable advantage over traditional bio-treatment systems (free-living bacteria) and good reusable performance. A better algicidal rate of 77.67% ± 1.14% at 7th day was obtained with the use of capsule embedding 50 mL of algicidal bacteria, enhanced by 11.05% comparing with same amount of free-living bacteria. Moreover, the algicidal rate of M. aeruginosa still reached 68.57% ± 2.88% after three times repetitive use. The effect of capsules on the fluorescence and antioxidant system of M. aeruginosa indicated that the photosystems were irreversibly damaged and the antioxidant response of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were significantly induced. Overall, capsules prepared in this study can provide a desirable environment for algicidal bacteria HL and ensure algicidal bacteria to in-situ work well in inhibiting booms of algae.

The reuse of waste glass for enhancement of heavy metals immobilization during the introduction of galvanized sludge in brick manufacturing.

The mixing of galvanized sludge in fired clay brick manufacturing has been regarded as an alternative approach for the consumption of galvanized sludge. Decreasing the surface area and porosity of fired brick definitely lowers the risk of heavy metal release. In this study, a novel method is proposed to reduce the surface area and porosity of bricks and promote heavy metal immobilization by adding waste glass. The introduction of waste glass enhanced the physical and mechanical performances of fired clay bricks and resulted in an increase in bulk density and compressive strength and a decrease in water absorption. Microstructure analysis showed that the texture of the bricks turned from porous to smooth and homogeneous due to the introduction of waste glass. Porosity analysis showed that surface area and pore volume of fired brick were substantially reduced. When the added waste glass amount exceeded 15 wt%, the heavy metal concentrations that leached from bricks containing 10 wt% galvanized sludge fired at 950 °C met the regulatory requirement. These results demonstrate that waste glass can be reused to enhance the stabilization/solidification of heavy metals, during the mixing of hazardous waste in bricks and ceramics manufacturing process.

Upcycling discarded polyethylene terephthalate plastics into superior tensile strength and impact resistance materials with a facile one-pot process.

Discarding PET plastic (dPET) causes serious environmental pollution and enormous fossil resources waste. Processing techniques have mainly focused on the conversion of dPET into monomers, with minimal reports highlighting their transformation into high-value materials. This work intends to transform dPET into a high-performance material with potential alternative value in harsh production environments. The soft and hard segments of the thermoplastic polyester elastomeric (TPEE) molecular structure are reacted and cross-linked with dPET using a facile one-pot process, and two main polymers, (C8H4O4)n and ((C16H18O4)0.76·(C4H8O)0.24)n are generated after the reaction. Through chemical reactions between TPEE and dPET, new characteristic products and chemical bond-crossing structures are formed, while the resulting product particles or multiple TPEE particles are anchored by the high viscosity of dPET, which endows the material with superior tensile strength (34.21 MPa) and impact resistance. The glass transition temperature (Tg) of the material implies that neither the molecular chain nor the chain segments can move, while only the atoms or groups composing the molecule vibrate at their equilibrium positions. The development of this new treatment method may contribute to the reduction of environmental pollution and the improvement of the high-value conversion and utilization of dPET.

Passivation mechanism of Cu and Zn with the introduction of composite passivators during anaerobic digestion of pig manure.

Heavy metals in livestock manure pose a threat to the environment after biogas fertilizer being utilized, while its bioavailability is reduced substantially by passivator during the anaerobic digestion. In this study, an optimal composite passivator of humic acid, fly ash and biochar with proportion of 7.5%:7.5%:7.5% and 5.0%:7.5%:7.5% is obtained and the passivation mechanism on Cu and Zn during anaerobic digestion of pig manure is explored. The content of humic acid (HA) in biogas residue increased by 15.66-27.82%, which promoted the transformation from FA-Cu/Zn to HA-Cu/Zn and was beneficial to the passivation of Cu and Zn. The bioavailability of Cu and Zn was reduced by the adsorption and complexation at the early and middle stages of anaerobic digestion. Humic substances play a major role in the passivation of heavy metals at the late stage. The composite passivator can improve the humification degree of biogas residue and reduce heavy metal biotoxicity.

Formation kinetics and reaction behavior of pentavalent chromium formed in the cement kiln co-processing of solid waste.

Cement kiln co-processing is becoming the main strategy to dispose of hazardous waste containing Cr. A newly-discovered pentavalent Cr compound, which was proved to be formed during cement kiln co-processing of solid waste, is partly responsible for the water-soluble Cr released from the cement. However, the formation characteristics and the solubility of Cr(V) are still unclear to date. In this study, the reaction kinetics and further redox reactions of Cr(V) at high temperature were examined, and its crystal structure and solubility were also explored. At the temperature range of 1000-1200 °C, the formation rate of Ca5(CrO4)3O0.5 reached over 90 % within 10 min, and then slowly increased to near 100 % from 10 min to 10 h. shows that Ca5(CrO4)3O0.5 is formed by interface reaction at an early period, and by diffusion at a later period. The kinetic analysis indicates that Ca5(CrO4)3O0.5 is initially formed through an interface reaction and subsequently through diffusion. Ca5(CrO4)3O0.5 was identified and assigned as hexagonal crystal group (P63/m). Approximately 0.55 g and 0.15 g of Ca5(CrO4)3O0.5 dissolve in neutral water at 100 °C and 50 °C, and the concentrations of Cr(V) in water reach 550 and 150 mg/L, respectively. Additionally, this study finds that at the temperature range of 400-700 °C Ca5(CrO4)3O0.5 can be oxidized into CaCrO4, and at the temperature higher than 1400 °C, it can be further converted into Ca3(CrO4)2 and reduced into CaCr2O4. This study gives a deep insight into Cr oxidation-reduction reaction during thermal treatment of solid waste. These insights provide a comprehensive understanding of Cr oxidation-reduction reactions during the thermal treatment of solid waste, offering valuable guidance for waste management strategies.

Preparation of a novel iron oxychloride (FeOCl) auxiliary electrode in promoting electrokinetic remediation of Cr(VI) contaminated soil: An experimental and DFT calculation analysis.

The utilization of auxiliary electrode can improve substantially the electrokinetic remediation efficiency of heavy metal contaminated soil. The increase in the auxiliary electrode performance is the key to further promote the electrokinetic remediation efficiency. In this study, two kinds of auxiliary electrodes, pure FeOCl and doped FeOCl with W and S, were prepared and used in the electrokinetic remediation of Cr(VI) contaminated soil. The system equipped with the auxiliary electrode doped FeOCl brought more stable system current (202 mA) and more uniform electric field than blank group (130 mA). The reduction rate of Cr(VI) was increased by 50% due to the presence of Fe2+ and S2-. The accelerating migration of ions by auxiliary electrode was responsible for the improvement in electrokinetic remediation efficiency. Density functional theory (DFT) calculation showed that Cl vacancy formation energies of pure FeOCl, S-doped FeOCl (S/FeOCl) and W-doped FeOCl (W/FeOCl) were 1.29, 1.15 and 1.49 eV respectively, and the ion diffusion barriers were 0.093, 0.099 and 0.148 eV respectively. Calculation results indicated that the doping of S was conducive to the diffusion of Cl ions, and the bonding of W-Cl was stronger than Fe-Cl. The charging and discharging process of auxiliary electrode became easier due to the formation of lower vacancy in S-doped FeOCl, which could bring a higher current for the electrokinetic remediation system. The electrochemical performance of FeOCl doped with W and S was improved obviously. This study provided a further explanation for the positive role of auxiliary electrode in electrokinetic remediation system.

A novel algicidal bacteria isolated from native snail lived in Taihu Lake against algal blooms: identification, degradation kinetic, and algicidal mechanism.

Harmful algal blooms (HABs) impacted negatively the water ecosystem, and produced toxic microcystins that poses toxic effect on liver, nervous, and genital system. The introduction of useful and adaptive algae-degrading microbes or bio-augmentation can be regarded as an efficient way to inhibit the outbreak of HABs. The purpose of this study is to evaluate the application potential of algicidal bacteria named XMC, which is isolated from native snails. Response surface methodology (RSM) experiments showed that self-characteristic and various external conditions affected the actual algae inhibition ability of XMC. In particular, actual algicidal efficiency was strongly depend on the temperature and growth stage of XMC, and the maximum algicidal rate could reach 93.95% within 7 days. The degradation curve of Microcystis aeruginosa was compliant with the first-order kinetic model, which could be used to predict the degradation effect of Microcystis aeruginosa in engineering applications. The analysis results of algae dissolution products showed that algicidal bacteria XMC had both direct and indirect algicidal capacity. In addition, XMC had strong algicidal ability and greater environmental adaptability, and its algae dissolution products were environmentally friendly. All results indicated that XMC had the potential to be used in the bio-degradation of cyanobacteria bloom.

Effects of Inorganic Passivators on Gas Production and Heavy Metal Passivation Performance during Anaerobic Digestion of Pig Manure and Corn Straw.

The treatment of livestock manure caused by the expansion of the breeding industry in China has attracted wide attention. Heavy metals in pig manure can pollute soil and water and even transfer to crops, posing harm to humans through the food chain. In this study, corn straw was selected as the additive and introduced into the anaerobic digestion. Sepiolite (SE), ferric oxide (Fe2O3), attapulgite (AT) and ferric sulfate (FeSO4) were used as passivators to compare the effects of these inorganic passivators on gas production and passivation of heavy metals during the process of the anaerobic digestion. When the dry mass ratio of pig manure to straw is 8:2, the gas production efficiency is optimal. SE, AT and ferric sulfate have a much stronger ability to improve gas production performance than Fe2O3. The total gas production increased by 10.34%, 6.62% and 4.56%, and the average methane production concentration increased by 0.7%, 0.3% and 0.4%, respectively. The influence of SE, AT and ferric sulfate on the passivation of heavy metals is much better than Fe2O3, and the fractions in biological effective forms of Cu and Zn reduced by 41.87 and 19.32%, respectively. The anaerobic digestion of mixed materials is conducive to the gas production and the passivation of heavy metals. Therefore, SE, AT and ferric sulfate are selected as composite passivators, and the optimal ratio of inorganic composite passivators i: AT 7.5 g/L, ferric sulfate 5 g/L and SE 7.5 g/L, according to the results of orthogonal experiments. This study can provide a theoretical basis for the safe application of biogas fertilizers.

Temperature dependent reduction of Cr(VI) to Cr(V) aroused by CaO during thermal treatment of solid waste containing Cr(VI).

Cr(VI) compounds at high temperature usually tend to decompose and reduce into Cr(III) due to thermodynamically instability for Cr(VI). This study found Cr(VI) could be reduced into Cr(V) instead of Cr(III) in the presence of CaO during heating solid waste containing Cr(VI). CaCrO4 is prepared and mixed with CaO as simulated solid waste containing Cr(VI). It was found that CaCrO4 reacted with CaO and formed a new product Ca5(CrO4)3O0.5 at temperature range of 800 and 1000 °C. The valence state of Cr in Ca5(CrO4)3O0.5 is determined to be +5 b y XPS analysis, and the color for new formed Cr(V) is observed in green, similar to Cr(III) compounds. The temperature and CaO are two keys to arouse the reduction reaction of Cr(VI) into Cr(V). In particular, the reduction of Cr(VI) into Cr(V) is strongly depended on temperature (800-1000 °C), this reaction can be balanced within 10 min, while prolonging sintering time has little help for promoting the reduction of Cr(VI) to Cr(V). Additionally, it was found Cr(V) can keep stable and not be re-oxidized into Cr(VI) at 800-1000 °C. Above results offers some new understanding and knowledge about the formation of Cr(V) in presence of much CaO or CaCO3 during heating solid waste containing Cr(VI).

Oxidation reaction behavior of Cr-hosting spinels during heating of solid wastes containing Cr.

Cr-hosting spinels are frequently formed during heating of solid wastes containing multiple metals, and its oxidation reaction (Cr(III) → Cr(VI)) is closely related with the toxicity of products. This study examined the reaction behaviors of Cr-hosting spinels (ZnCr2O4, CuCr2O4 and NiCr2O4) at high temperature and proposed possible oxidation mechanism. Cr-hosting spinels alone usually exhibit good thermal stability at high temperature. However, CaO can trigger the oxidation of Cr(III) in Cr-hosting spinels at 500-900 °C and ZnCr2O4 is easier to be oxidized than NiCr2O4 and CuCr2O4 at same condition. The oxidation of Cr-hosting spinels is accompanied with the formation of CaCrO4 and divalent metal oxides (ZnO, NiO and CuO). The broken and rebuilding of CrO bonds are key steps for Cr-hosting spinels oxidation, blocking the combination of free Cr with Ca and O atoms maybe more effective approach for suppressing Cr(III) oxidation. Furthermore, CaO can trigger the reduction of CaCrO4 into a new Cr(V) compound (Ca5(CrO4)3O0.5) at 900-1200 °C. As the temperature rising to 1300 °C, CuO reacts with CaCrO4 to form CuCrO2, in which Cu(II) and Cr(VI) are reduced into Cu(I) and Cr(III) respectively. This study provided some new knowledge for the reaction behavior of Cr-hosting spinels when solid wastes containing Cr were treated at high temperature.

Oxidation behavior of Cr(III) during thermal treatment of chromium hydroxide in the presence of alkali and alkaline earth metal chlorides.

The oxidation behavior of Cr(III) during the thermal treatment of chromium hydroxide in the presence of alkali and alkaline earth metal chlorides (NaCl, KCl, MgCl2, and CaCl2) was investigated. The amounts of Cr(III) oxidized at various temperatures and heating times were determined, and the Cr-containing species in the residues were characterized. During the transformation of chromium hydroxide to Cr2O3 at 300 °C approximately 5% of the Cr(III) was oxidized to form intermediate compounds containing Cr(VI) (i.e., CrO3), but these intermediates were reduced to Cr2O3 when the temperature was above 400 °C. Alkali and alkaline earth metals significantly promoted the oxidation of Cr(III) during the thermal drying process. Two pathways were involved in the influences the alkali and alkaline earth metals had on the formation of Cr(VI). In pathway I, the alkali and alkaline earth metals were found to act as electron transfer agents and to interfere with the dehydration process, causing more intermediate Cr(VI)-containing compounds (which were identified as being CrO3 and Cr5O12) to be formed. The reduction of intermediate compounds to Cr2O3 was also found to be hindered in pathway I. In pathway II, the alkali and alkaline earth metals were found to contribute to the oxidation of Cr(III) to form chromates. The results showed that the presence of alkali and alkaline earth metals significantly increases the degree to which Cr(III) is oxidized during the thermal drying of chromium-containing sludge.

The role of temperature on Cr(VI) formation and reduction during heating of chromium-containing sludge in the presence of CaO.

In this study, the temperature dependence of Cr(VI) formation and reduction in the presence of CaO was examined during the thermal treatment of sludge that contains chromium. thermogravimetry-differential scanning calorimetry and X-ray diffractometry were used to characterize the thermal behavior and phase transformation, respectively. Na2CO3 leaching procedure was employed to determine the amount of Cr(VI). The result showed that CaO promoted Cr(III) oxidation, however, its influence is very dependent on heating temperature, with the extent of the effect varying with temperature. From 200-400 °C, the presence of CaO facilitated formation of intermediate product Cr2O3+x containing Cr(VI) during dehydration of chromium hydrate, while Cr2O3+x would decompose as temperature over 400 °C, accompanied by part of Cr(VI) being reduced to Cr(III). From 500 to 900 °C, Cr(III) reacted with CaO to form a leachable CaCrO4 product. This product was stable and a prolonged heating time did not reduce the amount of Cr(VI) significantly. At 1000-1200 °C, part of CaCrO4 was reduced to Ca(CrO2)2 in 1h. While extended heating time above 1h resulted in the Ca(CrO2)2 being oxidized reversibly to CaCrO4 at 1200 °C. Since CaCrO4 is thermodynamically less stable over 1000 °C, MgO could induce CaCrO4 to be reduced into MgCr2O4 at around 900 °C, lower than that for the reduction from CaCrO4 into Ca(CrO2)2. It suggested that adding MgO might be a potential approach for inhibiting Cr(VI) formation during heating sludge containing chromium.

Stabilization of simulated lead sludge with iron sludge via formation of PbFe12O19 by thermal treatment.

This study investigated the feasibility of stabilizing lead sludge by reaction with iron sludge via the formation of PbFe12O19 through a thermal treatment process. Lead hydroxide was used to simulate lead-laden sludge and the sintering procedure was performed by firing a mixture of this simulated sludge together with iron sludge at a Fe/Pb molar ratio of 12 over the temperature range from 650 to 1400 °C. The accompanying phase transformations as well as the surface characteristic of sintered samples were observed by XRD and SEM, while the leaching behavior of the stabilized sludge in an acidic environment was evaluated by a modified Toxicity Characteristic Leaching Procedure (TCLP) test. The results confirmed that PbFe12O19 acts as a stabilization phase for lead, and showed that the formation of a PbFe12O19 phase began at 750 °C with the lead completely incorporated into the PbFe12O19 phase at 1050 °C. Above 1100 °C, the PbFe12O19 phase began to decompose, accompanied by the reappearance of Fe2O3. The volumes of compressed sludge samples were reduced significantly after thermal treatment, with accompanying volume reductions of 40% at 1050 °C. This study compared the leaching of lead from PbO and sintered sludge samples using a prolonged TCLP test, and the data showed that the PbFe12O19 phase was superior to the PbO and that the sintered sludge sample exhibited very high stability under acidic environments. These results suggest a promising and reliable method of reducing lead sludge mobility and toxicity has been identified.