Cr(III) removal from synthetic and industrial wastewaters by using co-gasification chars of rice waste streams

Recovery of rare earth elements (Nd3+ and Dy3+) by using carbon-based adsorbents from spent tire rubber

Two samples of spent tire rubber (rubber A and rubber B) were submitted to thermochemical conversion by pyrolysis process. A450, B450 and A900, B900 chars were obtained from rubber A and rubber B at 450 °C and 900 °C, respectively. The chars were then applied as recovery agents of Nd3+ and Dy3+ from aqueous solutions in mono and bicomponent solutions, and their performance was benchmarked with a commercial activated carbon. The chars obtained at 900 °C were the most efficient adsorbents for both elements with uptake capacities around 30 mg g-1. The chars obtained at 450 °C presented uptake capacities similar to the commercial carbon (≈ 11 mg g-1). A900 and B900 chars presented a higher availability of Zn ions that favored the ion exchange mechanism. It was found that Nd3+ and Dy3+ were adsorbed as oxides after Zn was released from silicate structures (Zn2SiO4). A900 char was further selected to be tested with Nd/Dy binary mixtures and it was found a trend to adsorb a slightly higher amount of Dy3+ due to its smaller ionic radius. The uptake capacity in bicomponent solutions was generally higher than for single component solutions due to the higher driving force triggered by the higher concentration gradient.

Food waste valorisation via gasification - A review on emerging concepts, prospects and challenges

Disposing of the enormous amounts of food waste (FW) produced worldwide remains a great challenge, promoting worldwide research on the utilization of FW for the generation of value-added products. Gasification is a significant approach for decomposing and converting organic waste materials into biochar, bio-oil, and syngas, which could be adapted for energy (hydrogen (H₂) and heat) generation and environmental (removal of pollutants and improving the soil quality) applications. Employment of FW matrices for syngas production through gasification is one of the effective methods of energy recovery. This review explains different gasification processes (catalytic and non-catalytic) used for the decomposition of unutilized food wastes and the effect of operating parameters on H₂-rich syngas generation. Also, potential applications of gasification byproducts such as biochar and bio-oil for effective valorization have been discussed. Besides, the scope of simulation to optimize the gasification conditions for the effective valorization of FW is elaborated, along with the current progress and challenges in the research to identify the feasibility of gasification technology for FW. Overall, this review concludes the sustainable route for conversion of unutilized food into hydrogen-enriched syngas production.

Multi-component adsorption study by using bone char: modelling and removal mechanisms

Highly efficient simultaneous removal of paracetamol and Cu²⁺ ions from aqueous solutions was accomplished by using bovine bone char (BC). The adsorption behaviour was determined by kinetic and equilibrium studies of both single and binary system solutions. BC is a predominantly mesoporous material with a surface area of 103 m² g^-1. The influence of the initial pH on Cu²⁺ removal was tested, suggesting that the optimal pH was 3.0. The removal of paracetamol from single and binary systems was 9.45 and 12.7%, respectively. On the other hand, the Cu²⁺ removal was 36.2% for a single system, suggesting a higher affinity for BC. Moreover, in the case of binary mixtures, the presence of paracetamol led to an enhanced affinity of Cu²⁺ due to a synergistic/cooperative mechanism, which led to a copper removal of 97.3%. The cooperative model was successfully adjusted to the equilibrium data of the binary systems. The modelling results indicated the formation of a first adsorption layer where paracetamol and copper are retained, and a second layer with a great affinity for copper ions after the formation of a Cu-paracetamol complex, leading to higher removal of Cu²⁺.

Versatile 3D reduced graphene oxide/poly(amino-phosphonic acid) aerogel derived from waste acrylic fibers as an efficient adsorbent for water purification

The fabrication of multifunctional materials to remove soluble heavy metal ions and dyes, as well as insoluble oils from waste water is urgently required, yet remains a daunting challenge because of difficulty in controlling their structure and property to satisfy various demands. Herein, for the first time, novel 3D reduced graphene oxide/poly(amino-phosphonic acid) (PAPA) aerogels (rGO/PAPAs) with different PAPA content were developed by solvothermal reduction of the graphene oxide and cross-linking with PAPA chain, and subsequently employed as versatile adsorbent for the removal of complex pollutants such as Cr(III) ion, methylene blue (MB) dye and various kinds of organic solvents from water. Benefiting from the synergistic effect of the reduced graphene oxide (rGO) sheet and PAPA component, as well as its unique 3D structure, the resultant aerogel (rGO/PAPA-2) gained amphiphilic, ultralight, and multifunctional properties. Thus, it showed a fast adsorption rate (within 15 min) and high adsorption capacity (up to 327.1 mg/g) for Cr(III) ion at an optimal pH of 5.5 due to its unique 3D network structure with abundant amino-phosphonic acid functional groups. The uptake of Cr(III) by rGO/PAPA-2 was fitted well with the Langmuir isotherm and pseudo-second-order kinetic model. The adsorption mechanism of Cr(III) onto rGO/PAPA-2 can be attributed to electrostatic attraction and surface complexation with APA groups. In addition, the rGO/PAPA-2 displayed an excellent adsorption performance for MB (694.5 mg/g) and several organic solvents (83.2 to 254.3 g/g). Moreover, the rGO/PAPA-2 exhibited a good regeneration (around 99%) and satisfactory recovery abilities for the tested adsorbates. Notably, PAPA chains can be easily prepared from waste acrylic fibers, making it become a cost effective but versatile candidate to prepare new material. Therefore, this work provides a new design strategy to fabricate the rGO/PAPA-2 aerogel with great prospect for sophisticated industrial wastewater cleanup.

Highly efficient porous carbons for the removal of W(VI) oxyanion from wastewaters

Pyrolysis chars derived from rice wastes were chemically activated and used in W(VI) oxyanion adsorption assays in synthetic and mining wastewaters. For comparison purposes, a commercial activated carbon (CAC) was also used. Different experimental conditions were tested in the adsorption assays: solid/liquid ratio (S/L), initial pH, contact time, and initial W concentration. The porous carbon P2C+KOH presented the overall best performance in both media, due to its high surface area (2610 m² g^-1), mesopore volume (1.14 cm³ g^-1), and neutral pH_pzc (6.92). In the synthetic wastewater, the highest uptake capacity of P2C+KOH (854 mg g^-1) was found in the assays with an S/L 0.1 g L^-1, an initial pH 2, and an initial W concentration of 150 mg L^-1, for 24 h. This value was almost 8 times higher than the one obtained for CAC (113 mg g^-1). In the mining wastewater, P2C+KOH showed an even higher uptake capacity (1561 mg g^-1) in the assay with the same experimental conditions, which was almost 3 times higher than for CAC (561 mg g^-1). These results suggest that P2C+KOH seems to be an efficient alternative to CAC in the W(VI) adsorption from liquid effluents.

Recovery of Cr(III) by using chars from the co-gasification of agriculture and forestry wastes

The aim of the present work was to assess the efficiency of biochars obtained from the co-gasification of blends of rice husk + corn cob (biochar 50CC) and rice husk + eucalyptus stumps (biochar 50ES), as potential renewable low-cost adsorbents for Cr(III) recovery from wastewaters. The two gasification biochars presented a weak porous structure (A_BET = 63-144 m² g^-1), but a strong alkaline character, promoted by a high content of mineral matter (59.8% w/w of ashes for 50CC biochar and 81.9% w/w for 50ES biochar). The biochars were used for Cr(III) recovery from synthetic solutions by varying the initial pH value (3, 4, and 5), liquid/solid (L/S) ratio (100-500 mL g^-1), contact time (1-120 h), and initial Cr(III) concentration (10-150 mg L^-1). High Cr(III) removal percentages (around 100%) were obtained for both biochars, due to Cr precipitation, at low L/S ratios (100 and 200 mL g^-1), for the initial pH 5 and initial Cr concentration of 50 mg L^-1. Under the experimental conditions in which other removal mechanisms rather than precipitation occurred, a higher removal percentage (49.9%) and the highest uptake capacity (6.87 mg g^-1) were registered for 50CC biochar. In the equilibrium, 50ES biochar presented a Cr(III) removal percentage of 27% with a maximum uptake capacity of 2.58 mg g^-1. The better performance on Cr(III) recovery for the biochar 50CC was attributed to its better textural properties, as well as its higher cation exchange capacity.

Removal of Cr(III) from aqueous solution by silica-gel/PAMAM dendrimer hybrid materials

Water pollution caused by Cr(III) is a serious environmental problem which bring adverse effect to environmental protection and public safety. Efficient removal of Cr(III) from aqueous solution is important for the remediation of Cr(III) pollution. Herein, a series of silica-gel/polyamidoamine (PAMAM) dendrimer hybrid materials (SG-G0~SG-G4.0) were used for the removal of Cr(III) from aqueous solution. The factors that affect the adsorption were extensively studied and the adsorption mechanism was demonstrated based on the experimental results and density functional theory (DFT) calculation. Result demonstrates the adsorption capacity of ester-terminated silica-gel/PAMAM dendrimers follow the order of SG-G2.5 > SG-G3.5 > SG-G1.5 > SG-G0.5, while that of amino-terminated ones decrease in the order of SG-G2.0 > SG-G4.0 > SG-G3.0 > SG-G1.0 > SG-G0. The highest adsorption is achieved at pH 4.0 for both ester- and amino-terminated materials. Adsorption kinetic indicates the adsorption equilibrium can be reached at about 240 and 180 min for amino- and ester-terminated hybrids, respectively. Adsorption kinetic can be well fitted by pseudo-second-order kinetic model with film diffusion process as the rate-limiting step. Adsorption isotherm follows Langmuir model with monolayer adsorption behavior. Fourier transform infrared spectra (FTIR) indicate the adsorption of Cr(III) by PAMAM dendrimer mainly involve the participation of N-H and C=O groups. DFT calculation demonstrates the uptake of Cr(III) by ester-terminated adsorbents mainly involves carbonyl oxygen and secondary amine nitrogen atoms to form tetra-coordinated chelate, while that of amino-terminated one tends to form hexa-coordinated chelates by carbonyl oxygen, primary and secondary amine nitrogen atoms.

Characterization and adsorption performance evaluation of waste char by-product from industrial gasification of solid refuse fuel from municipal solid waste

This study investigated the effect of steam and air flowrate combinations on the syngas efflux, physicochemical properties and adsorption performances (on congo red, CR and crystal violet, CV removal) of waste char by-product from the industrial gasification of solid refuse fuel from municipal solid waste. The BET surface area (11.4 m²/g), porosity (74.7%), fixed carbon content (25.8 wt%) and hydrophilicity (0.09) were enhanced with lower steam rate and higher air supply rate combination (MSWC-L) than for the higher steam rate and lower air supply rate combination (MSWC-H). Adsorption performances were higher for MSWC-L than MSWC-H on both CR (35.7-49.7 mg/g) and CV (235 to 356 mg/g) removal, suggesting that, higher air supply rate (214 Nm³/h; at 0.36 equivalence ratio) with lower steam rate (37 kg/h) were more effective gasification process conditions. Results showed that, syngas efflux was more sensitive to air supply rate than steam supply rate. Reactions in the combustion zone were not only limited to the pyrolysis gas vapours but to the char also. In conclusion, the waste chars from municipal solid waste gasification showed good potential as adsorbents in wastewater treatment.

The influence of temperature on the physicochemical properties of products of pyrolysis of leather-tannery waste

The present paper examines the pyrolysis of waste from leather tanneries at 300-500 °C. These studies are important because of difficulties in the utilisation of this type of waste as well as its energy potential as fuel. The pyrolysis of tannery waste and data from the relevant literature showed that thermal degradation can be explained using tanned collagen as a reference. Moreover, the experimental results indicated that this process is highly non-linear, due to various mechanisms of heat transport which cause temperature differences in a laboratory pyrolysis reactor. Thermogravimetric analysis has shown that the greater part of mass loss is observed between 80 and 500 °C and that the most significant mass release occurs at 325 °C. Moreover, the proportions of CO₂ and CO decrease along with increasing temperatures. The paper presents characteristics of the composition of solid, liquid, and gaseous products of leather-waste pyrolysis at various temperatures. The maximum heating value of gaseous products at 500 °C was 9.54 MJ/Nm³. An increase from 300 to 500 °C results in the dominant position of condensation polymerisation; the maximum value of the liquid phase yield is reached at 400 °C (42%). HHV analysis of the resulting char showed a maximum value of 21.18 MJ/kg at 450 °C. The results of oxidised component analysis showed that the major oxidised component of char was chromium oxide (Cr₂O₃), with a content of approximately 8.5% at all pyrolysis temperatures.