Separation and recycling of Au and Ag from waste light-emitting diodes.

Waste light-emitting diodes (LEDs) contain rare and precious metals which have attracted wide attention due to their high resource. In this study, experimental research was conducted on the separation and recycling of Au and Ag from LEDs. Firstly, thermal treatment and sieving were done to separate and enrich the metals in LEDs. With the constant heating rate of 10°C/min to 450°C under air atmosphere, the metals could be effectively separated from organics and the rare metals Au and Ag mainly concentrate in particles with a diameter ≤600 µm, whose concentration is about 1816 and 1429 mg/kg, respectively. Then, a mix-acid system of HCl-CH3COOH was introduced to leach Au and Ag from the enriched sample. The results show that the HCl-CH3COOH system could effectively leach Au and Ag, and the leaching performance of Au and Ag can reach 95.4% and 96.2%, respectively under the recommended conditions (total acid concentration 5 mol/L, HCl:CH3COOH = 4:1, leaching temperature 80°C, solid-liquid ratio 1:100, leaching time 5 h). The study can provide a new option for recycling of waste LEDs, which also provide a more environment-friendly method for Au and Ag leaching from industrial wastes.

Immobilization of heavy metals in biochar by co-pyrolysis of sludge and CaSiO3.

Sludge pyrolysis has become an important method of sludge recycling. Stabilizing heavy metals in sludge is key to sludge recycling. Currently, research on the co-pyrolysis of sludge and industrial waste is limited. This study aims to explore the impact and mechanism of the co-pyrolysis of sludge and CaSiO3 (the main component of slag) and to achieve the concept of "treating waste with waste". To this end, we added different proportions of CaSiO3 (0%, 3%, 6%, 9%, 12%, and 15%) for the co-pyrolysis with sludge, and varied the pyrolysis temperatures (300, 400, 500, 600, and 700 °C) and retention times (15, 30, 60, and 120 min) to study heavy-metal stabilization in sludge. Consequently, the optimum dosage of CaSiO3 required for the immobilization of different heavy metals was 9% (Cu, Zn, Pb, and Cr) and 15% (Ni). The contents of Cu, Zn, Pb, Cr, and Ni in the stable state (oxidized and residual states) were 92.73%, 79.23%, 99.55%, 92.43% and 90.33% respectively. At a pyrolysis temperature of 700 °C, the steady-state proportions of Cr, Pb, and Zn were 88.12%, 90.21%, and 77.21%, respectively. At a pyrolysis temperature of 400 °C, the stable-Cu and -Ni contents were 97.21% and 99.43%, respectively. The optimal dwelling time was 15 min. The results showed that the CaSiO3 addition weakened the O-H stretching vibration peak intensity, promoted the formation of aromatic and epoxy ring structures, and enhanced the heavy-metal immobilization. Furthermore, the CaSiO3 decomposition during co-pyrolysis produced SiO2, CaO, and Ca(OH)2, which helped stabilize heavy metals.

Fate and health risk assessment of heavy metals in Brassica chinensis L. (pak-choi) and soil amended by sludge-based biochar.

Biochar is widely used in agriculture to efficiently solve the problem of sludge. In this study, sludge-based biochar (referred to as BC1, BC2, and BC3) was prepared by mixing sludge with FeCl3, Na2SiO3, and Ca (H2PO4)2, respectively. Then, it was mixed with fresh soil to plant Brassica chinensis L. The analysis of the effects of the three biochar types showed that all of them were beneficial to the growth of Brassica chinensis L. We added the biochar to the soil and found that the concentration of heavy metals did not exceed the recommended threshold. Additionally, the aboveground part of Brassica chinensis L. met the standard requirement for food safety (GB 2761-2017). Notably, BC3 stood out with the best effect on the growth of Brassica chinensis L. and resulted in the improvement of the physical and chemical properties of soil such as ammonium nitrogen, available phosphorus, and available potassium (BC3 was followed by BC2 and BC1). BC3 could efficiently inhibit the migration of heavy metals, thereby reducing the overall heavy metal pollution level and ameliorating the soil nutrients. BC3 could increase the organic carbon by 258.92%, available phosphorus by 234.45%, and available potassium by 37.12% compared with the CK group. The THQ and TTHQ estimates of Brassica chinensis L. were lower than one, indicating that the health risk of heavy metal intake was not prominent. Additionally, the application of the proposed biochar could reduce the form of F1 (acid extracted state) and increase the form of F4 (residue state) in soil. Overall, we conclude that the application of the proposed biochar can promote the root absorption of heavy metals and inhibit the migration of heavy metals.

Co-pyrolysis of sewage sludge and Ca(H2PO4)2: heavy metal stabilization, mechanism, and toxic leaching.

The presence of unstable heavy metals in sewage sludge (SS) restricts its resource utilization. In this study, Ca(H2PO4)2 and SS were co-pyrolyzed to produce biochar, which contained relatively stable heavy metals. X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, and inductively coupled plasma atomic emission techniques were used to analyze the physical and chemical properties and heavy metal content of the biochar. The results indicated that co-pyrolysis of SS with Ca(H2PO4)2 resulted in the production of more stable heavy metals in the SS. The optimal co-pyrolysis conditions were a blended ratio of 15% Ca(H2PO4)2, 650 °C final temperature, 15 °C min-1, and 60 min retention time. The potential stabilization mechanisms of heavy metals were as follows: (1) organic decomposition and moisture (sourced from Ca(H2PO4)2 decomposition) evaporation resulted in greater biochar surface porosity; (2) phosphorous substances were complexed with heavy metals to form metal phosphates; and (3) the mixture reactions among inorganic substances, pyrolysis products of organics, and heavy metals resulted in the formation of highly aromatic metallic compounds. Additionally, the potential environmental risks posed by the heavy metals decreased from 65.73 (in SS) to 4.39 (in biochar derived from co-pyrolysis of SS and 15% of Ca(H2PO4)2). This study reports on a good approach for the disposal of SS and the reduction of its environmental risk.

Migration characteristics of heavy metals during simulated use of secondary products made from recycled e-waste plastic.

Recycling of plastics from e-waste can conserve resources, however, aging during the use of plastic products can cause the migration of heavy metals in additives. This study presents a methodology for evaluating the risks of heavy metals in waste plastic secondary products during long term use associated with heavy metal migration. The study processes were investigated by: (1) recycling waste plastics and producing secondary products; (2) thermal aging of secondary products; and (3) toxic leaching used to quantitatively analyse the dissolution of heavy metals. Combined with the changes in mechanical properties and microstructure, the effect of aging on the migration of heavy metals was observed. The results showed that the polymer appeared to delaminate, the adhesion of waste plastics to additives decreased, and the mechanical properties clearly decreased after the thermal aging experiment. Leaching experiments showed that the leached concentrations of Ni, Cu, Zn, Pb, and Sb in the three types waste plastic products increased over time. After 8 d of aging, the leached concentrations of Ni, Sb, and Pb exceeded the third, fourth, and third class of the groundwater quality standard, respectively. Specifically, the concentrations of Sb were 141, 289, and 21.1 times higher than the maximum permissible level. Therefore, management hierarchy and safe environmental recycling methods should be developed to reduce the risk of heavy metals in waste plastic secondary products.

Migration of heavy metal in electronic waste plastics during simulated recycling on a laboratory scale.

Recycling is the primary method to handle electronic waste plastics, however, little attention has been paid to the risk posed by heavy metal migration in waste plastic products. The effect of multistage recycling processes on heavy metal migration and the environmental risk posed by heavy metals during recycling processes were investigated by: (1) Recycling waste plastics and determining the heavy metal contents in secondary products; (2) Using toxic leaching experiments to assess environmental risks of heavy metal migration in secondary products; and (3) Evaluating the effect of recycling processes on the mechanical properties and microstructure of plastics. Results showed that the contents of some harmful heavy metals in processed products exceeded the Safety of Toys Standard. Toxic leaching tests showed that Ni, Cu, Zn, Pb, and Sb migrated outward during secondary products use. With increased recycling times, concentrations of migrated Ni, Cu, Zn, Pb, and Sb increased, and the leached concentrations exceeded the limits stipulated in the Groundwater Quality Standard. Increased recycling times also accelerated waste plastics aging and caused the deterioration of mechanical properties. Furthermore, adhesion between layers decreased, stratification and cracking in polymers appeared, and adhesion of waste plastics to additives decreased. Therefore, the environmental risks of waste plastic recycling should be carefully considered.

Improved bioleaching efficiency of metals from waste printed circuit boards by mechanical activation.

The low bioleaching efficiency of Acidithiobacillus ferrooxidans results in its sparse industrial application for metal extraction from waste printed circuit boards (WPCBs). To improve the bioleaching efficiency of Acidithiobacillus ferrooxidans, we propose the use of mechanical activation to dispose WPCBs prior to performing bioleaching. Response surface methodology (RSM), scanning electron microscope- energy dispersive spectrometer (SEM-EDS), and laser particle size analyzer (LPSA) were used to optimize and analyze the mechanical activation process, respectively. The optimal conditions for mechanical activation was a milling time of 2 h, milling speed of 340 r min-1, and ball material ratio (w/w) of 10/1; the bioleaching rates of Cu, Ni, and Zn were 94.33%, 90.69%, and 90.78%, respectively. The bioleaching rates of Cu, Ni, and Zn were 74.75%, 70.46%, and 71.05%, respectively, without mechanical activation pretreatment. SEM-EDS and LPSA analyses indicated that mechanical activation could lead to a smaller particle size and expose wrapped metals, thus improving the bioleaching efficiency oyf tyhe metals inside the WPCBs. The electrode potential of the metals was likely changed by the mechanical activation, resulting in an improvement of their bioleaching efficiency. Additionally, the bioleaching rates of Pb, Cr, and Cd after mechanical activation pretreatment were 10.29%, 74.89%, and 54.12%, respectively. Contrastingly, the bioleaching rates of Pb, Cr, and Cd without mechanical activation pretreatment were 5.18%, 59.97%, and 37.12%, respectively. Thereinto, the precipitation of PbSO4 may result in a decrease of leached Pb. We propose a mechanical activation process for improving the bioleaching efficiency of metals from WPCBs.

Thermal treatment of liquid crystal display panel scraps: The metals migration and potential environmental risk in solid residue.

Thermal treatment has been proved to be an efficient and promising method for organics removal from LCD panels and for resource recycling. Considering with the toxic metals contained in LCD panels and their potential risk, it is necessary to study and evaluate the metals behavior and potential risk associated with the thermal treatment of LCD panels. In this study, the migration and transformation behavior of ten metals (Cr, As, Al, In, Ni, Cu, Zn, Cd, Fe, Sn) in LCD panels were investigated during thermal treatment, as well as their potential environmental risk and leaching toxicity in solid residue were evaluated. Results showed that Cr, Ni, In, Cu and Fe exhibit obvious migration behavior from solid into gas phase/fly ash during thermal treatment, with the maximum migration rate of 52.8%, 54.7%, 37.7%, 30.8%, and 34.9% respectively under the experimental condition. Speciation transformation for the metals of Cr, Ni, In, Cu, Fe and Zn was also observed in solid residue after thermal treatment, which leads to the ecological risk increase of Cu, In and contamination risk increase of Fe. Meanwhile, the leachable concentration and leaching toxicity of Cu and Ni in solid residue showed increasing trend after thermal treatment. The results indicate that attentions should be paid on the emission control of Cr, Ni, In, Cu, Fe during thermal treatment of LCD panel scraps. Environmental risk of Cu, In, Fe and the leaching toxicity of Ni in solid residue after thermal treatment should also be concerned.

Pyrolysis-based separation mechanism for waste crystalline silicon photovoltaic modules by a two-stage heating treatment.

Heating treatment is the mainstream method to separate the modules in the waste photovoltaic (PV) module recycling process, which has not been studied thoroughly. In the present study, a two-stage heating treatment was conducted to separate the waste crystalline silicon solar panels. The TPT backing material could be recovered integrally by heating at 150 °C for 5 min, which was conducive to further recycling and regeneration. The poly(ethylene-co-vinyl) acetate (EVA) binder was removed by the pyrolysis process at the temperature of 500 °C; acetic acid and several hydrocarbon compounds were the main products of the pyrolysis process. Analysis showed that the pyrolysis of the EVA binder could be divided into two stages: deacetylation process (acetic acid formation) and long chain scission with radical reactions (hydrocarbon formation). Furthermore, the pyrolysis kinetics and pyrolysis mechanisms were studied based on the experimental data and sufficient theoretical foundation. Acetic acid was generated by the deacetylation process through a six-member cyclic transition state, and several hydrocarbon compounds were generated through a series of long chain scissions, free radical migrations and Diels-Alder cycloadditions. In this study, undamaged TPT backing materials, glass and silicon wafers were obtained, which could be recycled by further treatment. This study could perfect the process of waste crystalline silicon solar panel recycling and provide a fundamental basis for recycling the waste crystalline silicon solar panels in an environmentally friendly and efficient manner.

Stocks and environmental release of mercury in backlight cold cathode fluorescence lamps.

Cold cathode fluorescent lamps (CCFLs), with mercury as their essential component, were widely used as backlight in liquid crystal display (LCD) appliances before 2008. Since 2008, the mercury-free light emitting diode started to be used as a substitute for CCFLs and the replacement finished in about 2014. Nowadays, CCFLs are obsolete products from the viewpoint of manufacture but they are important as waste. In recent years, large amounts of CCFLs are flowing to waste phase for treatment and this has become a major issue in most countries. To better understand and control the risk of CCFLs, the stock of mercury in CCFLs, its flow to waste phase and mercury emission with the life cycle of CCFLs in mainland China were estimated in this study. Results showed that there was 15.2 tons of mercury stocked in CCFLs in main LCD appliances (i.e., LCD televisions, LCD monitors, and laptop monitors) from 2003-2015. CCFLs and mercury started to flow to waste phase around the year 2007 and will likely peak in 2018 with an annual flow of 324.8 million units and 1.5 tons respectively, then will likely decline dramatically till 2030. Dismantling and production were the two main life stages of CCFLs with mercury vapor release, during which approximately 2.1 tons and 1.2 tons of mercury were released to the atmosphere respectively. The research also indicates that mercury recycling in specialized facilities was another life stage with high mercury emission risk in which the processes of shredding, separation, and residue disposal are inevitably accompanied by mercury release.

Heavy metals in soil at a waste electrical and electronic equipment processing area in China.

For the objective of evaluating the contamination degree of heavy metals and analysing its variation trend in soil at a waste electrical and electronic equipment processing area in Shanghai, China, evaluation methods, which include single factor index method, geo-accumulation index method, comprehensive pollution index method, and potential ecological risk index method, were adopted in this study. The results revealed that the soil at a waste electrical and electronic equipment processing area was polluted by arsenic, cadmium, copper, lead, zinc, and chromium. It also demonstrated that the concentrations of heavy metals were increased over time. Exceptionally, the average value of the metalloid (arsenic) was 73.31 mg kg-1 in 2014, while it was 58.31 mg kg-1 in the first half of 2015, and it was 2.93 times and 2.33 times higher than that of the Chinese Environmental Quality Standard for Soil in 2014 and the first half of 2015, respectively. The sequences of the contamination degree of heavy metals in 2014 and the first half of 2015 were cadmium > lead > copper > chromium > zinc and cadmium > lead > chromium > zinc > copper. From the analysis of the potential ecological risk index method, arsenic and cadmium had higher ecological risk than other heavy metals. The integrated ecological risk index of heavy metals (cadmium, copper, lead, zinc, and chromium) and metalloid (arsenic) was 394.10 in 2014, while it was 656.16 in the first half of 2015, thus documenting a strong ecological risk.

Hydrothermal decomposition of liquid crystal in subcritical water.

Treatment of liquid crystal has important significance for the environment protection and human health. This study proposed a hydrothermal process to decompose the liquid crystal of 4-octoxy-4'-cyanobiphenyl. Experiments were conducted with a 5.7 mL stainless tube reactor and heated by a salt-bath. Factors affecting the decomposition rate of 4-octoxy-4'-cyanobiphenyl were evaluated with HPLC. The decomposed liquid products were characterized by GC-MS. Under optimized conditions i.e., 0.2 mL H2O2 supply, pH value 6, temperature 275°C and reaction time 5 min, 97.6% of 4-octoxy-4'-cyanobiphenyl was decomposed into simple and environment-friendly products. Based on the mechanism analysis and products characterization, a possible hydrothermal decomposition pathway was proposed. The results indicate that hydrothermal technology is a promising choice for liquid crystal treatment.

Estimating the impact of the home appliances trade-in policy on WEEE management in China.

The ever-increasing amount of waste electric and electronic equipment (WEEE) has become a global problem. In view of the deleterious effects of WEEE on the environment and the valuable materials that can be reused in them, many countries have focused their attention on the management of WEEE and the recovery technologies of WEEE. The Chinese government has been active in creating a legislative and institutional framework to realize WEEE recycling. In June 2009, Chinese government launched home appliances and electronics trade-in implementation solution. This paper elaborates the home appliances trade-in policy and its significant impact on the WEEE management. The trade-in policy is not only conducive to expanding the consumption demand and promoting the balance of domestic and overseas demand, but also favorable to improving the energy efficiency and reducing environmental pollution. Under this policy, China has successfully established an effective WEEE recycling system, using the financial means and network design. Experiences gained from the trade-in policy have shown that management systems of WEEE need to be designed and implemented in a multi-stakeholder dialogue.