The volatilization of heavy metals during co-combustion of food waste and polyvinyl chloride in air and carbon dioxide/oxygen atmosphere

Response of trace elements in urban deposition to emissions in a northwestern river valley type city: 2010-2021

Anthropogenic activities release significant quantities of trace elements into the atmosphere, which can infiltrate ecosystems through both wet and dry deposition, resulting in ecological harm. Although the current study focuses on the emission inventory and deposition of trace elements, their complex interactions remain insufficiently explored. In this study, we employ emission inventories and deposition data for eight TEs (Cr, Mn, Ni, Cu, Zn, As, Cd, Pb) in Lanzhou City to unveil the relationship between these two aspects. Emissions in Lanzhou can be roughly divided into two periods centered around 2017. Preceding 2017, industrial production constituted the primary source of TEs emissions except for As; coal combustion was the primary contributor to Cr, Mn, and As emissions; waste incineration played a significant role in As, Zn, and Cd emissions; biomass combustion influenced Cr and Cd emissions; and transportation sources were the predominant contributors to Pb and Cu emissions. With the establishment of waste-to-energy plants and the implementation of ultra-low emission retrofits, emissions from these sources decreased substantially after 2017. Consequently, emissions from industrial production emerged as the main source of TEs. The deposition concentrations of Cr, Mn, Ni, Cu, and Pb followed a similar trend to the emissions. However, Cd and As exhibited lower emissions and a less pronounced response relationship. Moreover, Zn concentrations fluctuated within a narrow range and showed a weaker response to emissions. The consistent changes in emissions and TEs deposition concentrations signify a shift in deposition pollution in Lanzhou city from Coal-fired pollution to that driven by transportation and industrial activities. Within this transition, the industrial production process offers significant potential for emission reduction. This insight provides a crucial foundation for managing TEs pollution and implementing strategies to prevent ecological risks.

A global pollutant (PVC-polyvinyl chloride) applied as heavy metal binder from aqueous samples: green principles from synthesis to application

We have developed a clean route for the modification of polyvinylchloride surface (PVC) with 4-amino-5-hydrazino-1,2,4-triazole-3-thiol molecule. The modification reaction was investigated through Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analysis. According to our findings, S-H groups are responsible to the molecule attachment and nitrogen atoms are directly involved in metal ion coordination. These results are in agreement with the pseudo-second-order kinetic model, which infers that chemisorption is the main mechanism for metal removal. Adsorption isotherms of Cd(II), Cu(II) and Pb(II) follow the Langmuir model and the results indicated that N_s values are 0.39, 0.52 and 0.15 mmol g^-1, respectively. The calculated Ø^max values for Cu(II), Pb(II) and Cd(II) were 3.93, 2.95 and 1.13, respectively, indicating that three types of complex are formed depending on the adsorbed species. Therefore, it can be concluded that PVC use as adsorbent is feasible since it requires a simple modification reaction with nontoxic and low-cost solvents.

Ash-to-emission pollution controls on co-combustion of textile dyeing sludge and waste tea

Given the globally increased waste stream of textile dyeing sludge (TDS), its co-combustion with agricultural residues appears as an environmentally and economically viable solution in a circular economy. This study aimed to quantify the migrations and chemical speciations of heavy metals in the bottom ashes and gas emissions of the co-combustion of TDS and waste tea (WT). The addition of WT increased the fixation rate of As from 66.70 to 83.33% and promoted the chemical speciation of As and Cd from the acid extractable state to the residue one. With the temperature rise to 1000 °C, the fixation rates of As, Cd, and Pb in the bottom ashes fell to 27.73, 8.38, and 15.40%, respectively. The chemical speciation perniciousness of Zn, Cu, Ni, Mn, Cr, Cd, and Pb declined with the increased temperature. The ash composition changed with the new appearances of NaAlSi₃O₈, CaFe₂O₄, NaFe(SO₄)₂, and MgCrO₄ at 1000 °C. The addition of WT increased CO₂ and NO_x but decreased SO₂ emissions in the range of 680-1000 °C. ANN-based joint optimization indicated that the co-combustion emitted SO₂ slightly less than did the TDS combustion. These results contribute to a better understanding of ash-to-emission pollution control for the co-combustion of TDS and WT.

Optimization of biochar production based on environmental risk and remediation performance: Take kitchen waste for example

Two major obstacles that need to be addressed for environmental application of biochar include its environmental risk and remediation performance for target pollutants. In this study, kitchen waste was taken as an example to optimize the pyrolysis temperature for biochar production based on its heavy metal risk and Cd(II) remediation performance. The results showed that the pH and ash content of kitchen waste biochar (KWB) increased; however, the yield, H/C, and N/C decreased with increasing pyrolysis temperature. Total content of heavy metals in KWB got enriched after pyrolysis, while heavy metals' risk was reduced from moderate to low due to the transformation of directly toxic heavy metal fractions into potentially and/or non-toxic fractions. The equilibrium adsorption capacities of biochar for Cd(II) ranked as follows: 49.0 mg/g (600 °C), 46.5 mg/g (500 °C), 23.6 mg/g (400 °C), 18.2 mg/g (300 °C). KWB pyrolyzed at 500 °C was found to be the most suitable for green, efficient, and economic remediation of Cd(Ⅱ) contaminated water. SEM-EDS and XPS characterization results indicated that KWB removed Cd(II) via precipitation, complexation with carboxyl/hydroxyl, ion exchange with metal cations, and coordination with π-electrons. This study puts forward a new perspective for optimizing biochar production for environmental application.

Volatilization characteristics and risk evaluation of heavy metals during the pyrolysis and combustion of rubber waste without or with molecular sieves

The volatilization characteristics and risk evaluation of heavy metals (As, Cd, Co ,Cr, Cu, Ni, Pb, Zn, and Ti) during pyrolysis and combustion of rubber waste with or without molecular sieves (MS) were studied. The addition of MS during pyrolysis inhibited the volatilization of As and promoted the volatilization of Ni and Co, while during combustion it inhibited the volatilization of Pb, Zn and promoted the volatilization of Cu. For Cd, Cr, Ni, Zn and Ti, their volatilization rates during pyrolysis were significantly higher than those during combustion, whereas for As and Cu, the volatilization rates during pyrolysis were lower than those during combustion. Risk evaluation of gaseous heavy metals was exhibited based on the Potential Ecological Risk Index (RI) method. The potential ecological risk during combustion was generally lower than that during pyrolysis. The research results provide important information of heavy metals control during waste thermal treatment.

Effects of natural and modified calcium-based sorbents on heavy metals of food waste under oxy-fuel combustion

Performance of natural and modified calcium-based sorbents for heavy metals for food waste under oxy-fuel combustion in a lab-scale tubular furnace was carried out. The effects of furnace temperature, sorbents type, and CO₂/O₂ ratio on adsorption of heavy metals were investigated. Increasing the furnace temperature helped fixing Al in the bottom ash, but increased the volatilization of Zn. The results showed that heavy metals captured by sorbents highly depended on the metals types. Nature and modified CaO had excellent performance for Al capture while CaCO₃ could not absorb Al. Neither CaCO₃ or CaO could not use as sorbents for the Cr capture. CO₂/O₂ ratio highly affected the capture of Cr and Zn but had no influence on Al, and the decrease of CO₂/O₂ ratio would help capturing Cr and Zn. This work contributes to the heavy metals controlled by Ca-based sorbents during municipal solid waste oxy-fuel combustion.

High-temperature chlorination of PbO and CdO induced by interaction with NaCl and Si/Al matrix

Municipal solid-waste incineration leads to emission of lead (Pb) and cadmium (Cd), which vaporize in furnace and condense in flue. NaCl in waste has been proven to enhance volatilization of Pb and Cd at high temperatures via chlorination of oxides to chlorides; however, this process was not well-understood so far due to its complexity. This study decoupled the indirect chlorination process and direct chlorination process so that these two processes were investigated separately. A horizontal tube furnace was used to heat the mixtures of NaCl and Si/Al matrix for indirect chlorination and the mixtures of NaCl, PbO/CdO and Si/Al matrix for direct chlorination. A set of dynamic sampling devices was designed and used to obtain dynamic data during temperature rising. The indirect chlorination process was initiated above 800 °C in O₂ + H₂O atmosphere and O₂ atmosphere and above 1000 °C in N₂ atmosphere. Al₂O₃ exhibited higher activity than SiO₂ to react with NaCl, releasing HCl or Cl₂. In the Cl release reaction, NaCl was in the gas phase. The direct chlorination process was initiated at 650–700 °C when the Si/Al matrix contained SiO₂ only and at around 800 °C when the Si/Al matrix contained Al₂O₃ only or both SiO₂ and Al₂O₃. SiO₂ exhibited higher activity than Al₂O₃ in direct chlorination. The pre-reaction between PbO/CdO and Si/Al matrices was considered as the necessary condition for direct chlorination. During chlorination in O₂ + H₂O atmosphere, indirect chlorination and direct chlorination occurred simultaneously, and the latter dominated the volatilization of Pb and Cd.

The chlorination process by NaCl was decoupled as indirect chlorination and direct chlorination, which were investigated separately.

Emissions of toxic pollutants from co-combustion of demolition and construction wood and household waste fuel blends

Four different types of fuel blends containing demolition and construction wood and household waste were combusted in a small-scale experimental set-up to study the effect of fuel composition on the emissions of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), chlorobenzenes (PCBzs), chlorophenols (PCPhs) and polycyclic aromatic hydrocarbons (PAHs). Two woody materials, commercial stemwood (ST) and demolition and construction wood (DC) were selected because of the differences in their persistent organic pollutants (POPs), ash and metals content. For household waste, we used a municipal solid waste (MSW) and a refuse-derived fuel (RDF) from MSW with 5-20 wt% and up to 5 wt% food waste content respectively. No clear effect on the formation of pollutants was observed with different food waste content in the fuel blends tested. Combustion of ST-based fuels was very inefficient which led to high PAH emissions (32 ± 3.8 mg/kg_fuel). The use of DC clearly increased the total PCDD and PCDF emissions (71 ± 26 μg/kg_fuel) and had a clear effect on the formation of toxic congeners (210 ± 87 ng WHO₂₀₀₅-TEQ/kg_fuel). The high PCDD and PCDF emissions from DC-based fuels can be attributed to the presence of material contaminants such as small pieces of metals or plastics as well as timber treated with chromated copper arsenate preservatives and pentachlorophenol in the DC source.