Adding value to gasification and co-pyrolysis chars as removal agents of Cr3

High efficient COVID-19 waste co-pyrolysis char/TiO2 nanocomposite for photocatalytic reduction of Cr(VI) under visible light

Titanium dioxide (Titania) nanoparticle-coated biochar derived through co-pyrolysis of COVID-19 waste face mask (WFM) and Moringa oleifera seed cake (MO) provides an effective way to alleviate toxic metal in wastewater. This study investigates the effects of Biochar/titania photocatalyst preparation, characterization, and its photoreduction of Cr(VI). The morphological and functional modifications in the catalyst were identified using X-Ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, ultraviolet spectrophotometer, surface area analysis, and Raman spectrophotometer, respectively. The influencing parameters, namely, pH, photocatalyst dosage, initial pollutant concentration, and visible light irradiation time, have been investigated. The findings reveal that the Cr(VI) reduction by the photocatalyst was highly facilitated by photocatalytic process. The prepared photocatalyst shows higher and faster reduction rate of Cr(VI) and also improves the catalyst stability. The photoreduction of Cr(VI) ensembles well with pseudo-first order kinetics. At 180 min of reaction time, maximum Cr(VI) reduction of 98.65% was achieved at pH 2, 0.3 g/L catalyst dosage, and 10 ppm initial concentration, respectively. The synthesized photocatalyst shows excellent recycling performance up to 7 times, and these studies proved that the prepared catalyst is cost-effective and efficiently employed for removing pollutants.

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.

Application of co-pyrolysis biochar for the adsorption and immobilization of heavy metals in contaminated environmental substrates

Heavy metal pollution has been considered as a serious threat to the environment and human in the past decades due to its toxic and unbiodegradable properties. Recently, extensive studies have been carried out on the removal of heavy metals, and various adsorption materials have been successfully developed. Among, biochar is a promising option because of its advantages of various biomass sources, abundant microporous channels and surface functional groups, as well as its attractive economic feasibility. However, the application of pristine biochar is limited by its low adsorption capacity and nonregenerative property. Co-pyrolysis biochar, produced from the pyrolysis of biomass with the addition of another biomass or non-biomass precursor, is potential in overcoming the limitation of pristine biochar and achieving superior performance for heavy metal adsorption and immobilization. Therefore, this article summarizes the recent advances in development and applications of co-pyrolysis biochar for adsorption and immobilization of various heavy metals in contaminated environmental substrates. In details, the production, characteristics and advantages of co-pyrolysis biochar are initially presented. Subsequently, the adsorption behaviors and mechanisms of different heavy metals (including Hg, Zn, Pb, Cu, Cd, Cr, As, etc.) in flue gas and wastewater by co-pyrolysis biochar are reviewed, as well as factors influencing their adsorption capacities. Meanwhile, the immobilization of heavy metals in both biochar itself and contaminated soils by co-pyrolysis biochar is discussed. Finally, the limitations of current studies and future prospects are proposed. It aims at providing a guideline for the exploitation and application of cost-effective and environmental-friendly co-pyrolysis biochar in the decontamination of environmental substrates.

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.

State-of-the-Art Char Production with a Focus on Bark Feedstocks: Processes, Design, and Applications

In recent years, there has been a surge of interest in char production from lignocellulosic biomass due to the fact of char’s interesting technological properties. Global char production in 2019 reached 53.6 million tons. Barks are among the most important and understudied lignocellulosic feedstocks that have a large potential for exploitation, given bark global production which is estimated to be as high as 400 million cubic meters per year. Chars can be produced from barks; however, in order to obtain the desired char yields and for simulation of the pyrolysis process, it is important to understand the differences between barks and woods and other lignocellulosic materials in addition to selecting a proper thermochemical method for bark-based char production. In this state-of-the-art review, after analyzing the main char production methods, barks were characterized for their chemical composition and compared with other important lignocellulosic materials. Following these steps, previous bark-based char production studies were analyzed, and different barks and process types were evaluated for the first time to guide future char production process designs based on bark feedstock. The dry and wet pyrolysis and gasification results of barks revealed that application of different particle sizes, heating rates, and solid residence times resulted in highly variable char yields between the temperature range of 220 °C and 600 °C. Bark-based char production should be primarily performed via a slow pyrolysis route, considering the superior surface properties of slow pyrolysis chars.

High-Efficiency Removal of Cr(VI) from Wastewater by Mg-Loaded Biochars: Adsorption Process and Removal Mechanism

Biochars were produced with magnesium chloride as an additive for the sorption of hexavalent chromium dissolved in water using five types of straw (from taro, corn, cassava, Chinese fir, and banana) and one type of shell (Camellia oleifera) as the raw materials. The removal of hexavalent chromium by the six biochars mainly occurred within 60 min and then gradually stabilized. The kinetics of the adsorption process were second order, the Langmuir model was followed, and the adsorption of Cr(VI) by the six biochars was characterized by Langmuir monolayer chemisorption on a heterogeneous surface. Banana straw biochar (BSB) had the best performance, which perhaps benefitted from its special structure and best adsorption effect on Cr(VI), and the theoretical adsorption capacity was calculated as 125.00 mg/g. For the mechanism analysis, Mg-loaded biochars were characterized before and after adsorption by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), and scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS). The adsorption mechanism differed from the adsorption process of conventional magnetic biochar, and biochar interactions with Cr(VI) were controlled mainly by electrostatic attraction, complexation, and functional group bonding. In summary, the six Mg-loaded biochars exhibit great potential advantages in removing Cr(VI) from wastewater and have promising potential for practical use, especially BSB, which shows super-high adsorption performance.

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.

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.

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

Blends of rice waste streams were submitted to co-gasification assays. The resulting chars (G1C and G2C) were characterized and used in Cr(III) removal assays from a synthetic solution. A Commercial Activated Carbon (CAC) was used for comparison purposes. The chars were non-porous materials mainly composed by ashes (68.3-92.6% w/w). The influences of adsorbent loading (solid/liquid ratio - S/L) and initial pH in Cr(III) removal were tested. G2C at a S/L of 5 mg L^-1 and an initial pH of 4.50 presented an uptake capacity significantly higher than CAC (7.29 and 2.59 mg g^-1, respectively). G2C was used in Cr(III) removal assays from an industrial wastewater with Cr(III) concentrations of 50, 100 and 200 mg L^-1. Cr(III) removal by precipitation (uptake capacity ranging from 11.1 to 14.9 mg g^-1) was more effective in G2C, while adsorption (uptake capacity of 16.1 mg g^-1) was the main removal mechanism in CAC.

Study of the removal mechanism of aquatic emergent pollutants by new bio-based chars

This work is dedicated to study the potential application of char byproducts obtained in the gasification of rice husk (RG char) and rice husk blended with corn cob (RCG char) as removal agents of two emergent aquatic contaminants: tetracycline and caffeine. The chars presented high ash contents (59.5-81.5%), being their mineral content mainly composed of silicon (as silica) and potassium. The samples presented a strong basic character, which was related to its higher mineral oxides content. RCG char presented better textural properties with a higher apparent surface area (144 m² g^-1) and higher micropore content (V _micro = 0.05 cm³ g^-1). The alkaline character of both chars promoted high ecotoxicity levels on their aqueous eluates; however, the ecotoxic behaviour was eliminated after pH correction. Adsorption experiments showed that RG char presented higher uptake capacity for both tetracycline (12.9 mg g^-1) and caffeine (8.0 mg g^-1), indicating that textural properties did not play a major role in the adsorption process. For tetracycline, the underlying adsorption mechanism was complexation or ion exchange reactions with the mineral elements of chars. The higher affinity of RG char to caffeine was associated with the higher alkaline character presented by this char.

Preparation of dithiocarbamate polymer brush grafted nanocomposites for rapid and enhanced capture of heavy metal ions

A core-brush nanocomposite for rapid and enhanced adsorption of heavy metal ions was explored by combining SI-ATRP and DTC functionalization.