Stabilization of heavy metals in solid waste and sludge pyrolysis by intercalation-exfoliation modified vermiculite.

Increasing amounts of solid waste and sludge have created many environmental management problems. Pyrolysis can effectively reduce the volume of solid waste and sludge, but there is still the problem of heavy metal contamination, which limits the application of pyrolysis in environmental management. The intercalated-exfoliated modified vermiculite (IEMV) by intercalators of sodium dodecylbenzene sulfonate, hexadecyltrimethylammonium bromide and octadecyltrimethylammonium bromide were used to control the release of Cd, Cr, Cu, Zn and Pb during pyrolysis process of sludge or solid waste. The retention of heavy metals in sludge was generally better than that in solid waste. The IEMV by octadecyltrimethylammonium bromide as the intercalator calcined 800 °C (STAB-800) was the best additive for heavy metal retention, and the retention of Cr, Cu and Zn was significantly better than that of Pb and Cd. Cr, Cu, Zn and Pb were at low risk, while Cd had considerable risk under certain circumstances. New models were proposed to comprehensively evaluate the results of the risk and forms of heavy metals, and the increasing temperature was beneficial in reducing the hazards of heavy metals by the addition of STAB-800. The reaction mechanism of heavy metals with vermiculite was revealed by simulation of reaction sites, Fukui Function and Frontier Molecular Orbital. Thermal activation-intercalated-exfoliated modified vermiculite (T-IEMV) is more reactive and had more active sites for heavy metals. Mg atoms and outermost O atoms are the main atoms for T-IEMV to react with heavy metals. The Cr, Cu and Zn have better adsorption capacity by T-IEMV than Pb and Cd. This study provides a new insight into managing solid waste and sludge and controlling heavy metal environmental pollution.

Effective stabilization of heavy metals in solid waste and sludge pyrolysis using intercalated-exfoliated modified vermiculite: Experiment and simulation study.

Pyrolysis is effective in reducing the volume of solid waste and sludge, and produces less pollutants than incineration and landfill, but the process still suffers from heavy metal pollution. Four types of intercalated-exfoliated modified vermiculite (UIV, DIV, TIV and 3IV) were prepared using urea, dimethylsulfoxide, tributyl phosphate and 3-aminopropyltriethoxysilane as intercalators for the control of Cd, Cr, Cu, Pb and Zn in municipal sewage sludge (MSL), paper mill sludge (PML), municipal domestic waste (MWA) and aged refuse (AFE). The larger the interlayer spacing of the vermiculite, the more favorable the retention of heavy metals. 3IV was the most effective additive, with an average retention of more than 75 % of all heavy metals at 450 ℃ for the four raw materials. Cr, Cu, Pb and Zn were all at low potential ecological risk (Pr), while Cd was moderate or considerable Pr, and the addition of 3IV reduced the Pr. Distribution of intercalators between vermiculite interlayers was haphazard, and interlayer spacing results were close to those of the experiment (except for tributyl phosphate). The reactive electrons mainly flowed from the Highest Occupied Molecular Orbital (HOMO) of vermiculite flakes to the Lower Unoccupied Molecular Orbital (LUMO) of heavy metal chlorides. In contrast, the reactive electrons mostly flowed from the HOMO of heavy metal oxides to the LUMO of vermiculite flakes. Heavy metal oxides were more readily adsorbed on vermiculite flakes than heavy metal chlorides, and the adsorption capacity of Cr and Zn was stronger than that of Cd, Pb and Cu.

Fabrication of porous carbon derived from cotton/polyester waste mixed with oyster shells: Pore-forming process and application for tetracycline removal.

Porous carbon was fabricated from cotton/polyester-based textile wastes as a carbon source coupled with oyster shells for tetracycline removal. The preparation conditions were optimized and detailed characterization was conducted to study the effects of oyster shells on cotton/polyester pyrolysis. The optimal pyrolysis temperature (900 °C), pyrolysis time (1 h) and mass ratio (OS/CPW of 1:1) were determined using the Box-Behnken experiment. The best porous carbon reached a surface area of 645.05 m2/g. Oyster shells acted as templates to produce cotton/polyester-based porous carbon and a possible pore-forming process was proposed. CaO was converted from CaCO3, which played the dominant role in developing the mesoporous structure. CO2 gas released from CaCO3 promoted the creation of micropore structure. In addition, the impurites of oyster shells acted as the dispersing agent inhibiting CaCO3 and CaO aggregation and growth. Fe2O3 and K2O from impurities reacted with the carbon skeleton to increase microporosity. Finally, the well-developed and uniform porous carbon was obtained. The first-pseudo order model and Langmuir isotherms were suitable. The maximum adsorption capacity of PC-OS-900 was 515.17 mg/g which competed with other waste-based adsorbents. The TET adsorption mechanism was related to pore distribution, hydrogen bonds, π-π EDA interactions and electrostatic interactions.

Conversion of cotton textile waste to clean solid fuel via surfactant-assisted hydrothermal carbonization: Mechanisms and combustion behaviors.

The cotton textile was an abundant energy resource while was otherwise treated as waste. In this work, surfactants were used as catalysts in the hydrothermal carbonization (HTC) to transform cotton textile waste (CTW) into clean solid fuel. Furthermore, the conversion mechanisms of hydrothermal products during surfactant-assisted HTC were preliminarily proposed. The results showed that Span 80 and sodium dodecylbenzenesulfonate facilitated the transformation of CTW into bio-oil, while Tween 80 was more conducive to the development of pseudo-lignin, which endowed hydrochars higher energy density and updated the fuel quality and combustion behavior. Therefore, the research presented an effective method to convert CTW to clean solid fuel through the HTC treatment combining with surfactants.

Effect of dispersant on the synthesis of cotton textile waste-based activated carbon by FeCl2 activation: characterization and adsorption properties.

Considering the accumulation and high consumption of activating agents, anhydrous ethanol (AE) could be used to dissolve them to improve the dispersion effect, which was an effective way of improving the practical utilization rate. In this study, FeCl2 was dissolved in AE and further impregnated cotton textile waste (CTW) to prepare activated carbons (ACs) by pyrolysis. Afterward, ACs prepared in optimal conditions determined by the orthogonal experiment evaluated the physicochemical properties and adsorption capacities for Cr(VI). The results illustrated that AE greatly increased the dispersion of FeCl2 on CTW, reduced the conventional impregnation dosage, and remarkably improved the activation efficiency. Textural analyses revealed that ACs exhibited excellent porosity properties and graphite carbon structure. FeCl2 catalyzed the decomposition of volatile substances to produce gaseous products and promoted the transformation of amorphous carbon to graphite carbon that was conducive to pore development, followed by the formation of developed micropores and crystal structures. The adsorption performance of ACs was estimated using Cr(VI), and the adsorption was fitted with the pseudo-second-order kinetic and the Langmuir isotherm. Furthermore, the ACs possessed superior magnetization and reusability. Graphical abstract.

Synthesis of char-based adsorbents from cotton textile waste assisted by iron salts at low pyrolysis temperature for Cr(VI) removal.

Char-based adsorbents (char-FeCl3, char-FeCl2, and char-FeCit) derived from cotton textile waste (CTW) were synthesized by one-step low-temperature pyrolysis approach with different iron salts. The properties of the samples were conducted by BET, SEM, EDS, XRD, XPS, TEM, and FTIR. The results suggested that the surface areas of char-FeCl3 and char-FeCl2 were higher than those of char-FeCit. The presence of Fe2O3 as well as pyrolysis gas (HCl (g) and H2O (g)) could catalyze the formation of porosity. Meanwhile, FeCl3 showed the strongest catalysis effect to decompose cellulose to produce char. The pyrolysis process analysis was investigated by means of thermogravimetry-DSC. FeCl3 and FeCl2 could accelerate the breakage of cellulose structure whereas FeC6H5O7 was not beneficial to form char at low temperature as the incomplete decomposition of citrate. The adsorption property of Cr(VI) for the chars was evaluated. Adsorption processes were fitted well with the Freundlich model, and char-FeCl3 presented the best adsorptive capacity (70.39 mg/g). Thus, this low-temperature pyrolysis method was economical and technologically simplified as well as efficient adsorption capacity of Cr(VI) removal. Graphical abstract.