A review on control factors of pyrolysis technology for plants containing heavy metals

Biochar from phytoremediation plant residues: a review of its characteristics and potential applications

Phytoremediation is a cost-effective and eco-friendly plant-based approach promising technique to repair heavy metal-contaminated soils. However, a significant quantity of plant residues needs to be properly treated and utilized. Pyrolysis is an effective technology for converting residues to biochar, which can solve the problem and avoid secondary contamination. This paper reviews the generation, and physicochemical properties of biochar from phytoremediation residues, and its application in soil improvement, environmental remediation, and carbon sequestration. In spite of this, it is important to be aware of the potential toxicity of heavy metals in biochar and the environmental risks of biochar before applying it to practical applications. Future challenges in the production and application of residue-derived biochar include the rational selection of pyrolysis parameters and proper handling of potentially hazardous components in the biochar.

Stability and ecological risk assessment of nickel (Ni) in phytoremediation-derived biochar

Improper treatment of heavy metal-enriched biomass generated after phytoremediation might cause secondary pollution in soil and water. At present, the pyrolysis process is an effective method for the treatment of phytoremediation residue. In this study, Ni-enriched biomass was prepared using hydroponics method and further pyrolyzed at different temperatures (300-700 °C). At low pyrolysis temperatures (below 500 °C), carbonate precipitation was the main reason of Ni stabilization in biochar. Nevertheless, the formed phosphate and aluminosilicate were important factors for immobilizing Ni in biochar at high pyrolysis temperatures (above 500 °C). Moreover, the oxidizable (F3) and residual (F4) components of Ni in biochar increased with increasing pyrolysis temperature, which indicated that higher pyrolysis temperature could effectively reduce the bioavailability of Ni in biochar. The results of deionized water, acidification, oxidation, and toxic characteristic leaching procedure (TCLP) experiments showed that pyrolysis temperature was the dominant factor for Ni stabilization in biochar. The ecological risk assessments further proved that pyrolyzed Ni-enriched biochar could reduce the environmental toxicity and potential ecological risks of Ni. In the soil simulated experiment, the soil microenvironment gradually promoted the transformation of Ni in BCNiX from bioavailable fraction to stable fraction. Overall, this study would expose more reasonable reference for the long-term storage of phytoremediation residues.

A review on the emerging conversion technology of cellulose, starch, lignin, protein and other organics from vegetable-fruit-based waste

A large amount of vegetable-fruit-based waste (VFBW) belonging to agricultural waste is produced around the world every year, imposing a huge burden on the environment and sustainable development. VFBW contains a lot of water and useful organic compounds (e.g., cellulose, minerals, starch, proteins, organic acids, lipids, and soluble sugars). Taking into account the composition characteristics and circular economy of VFBW, many new emerging conversion technologies for the treatment of VFBW (such as hydrothermal gasification, ultrasound-assisted extraction, and synthesis of bioplastics) have been developed. This review summarizes the current literature discussing the technical parameters, process, mechanism, and characteristics of various emerging conversion methods, as well as analyzing the application, environmental impact, and bio-economy of by-products from the conversion process, to facilitate solutions to the key problems of engineering cases using these methods. The shortcomings of the current study and the direction of future research are also highlighted in the review.

Effect of Cd on Pyrolysis Velocity and Deoxygenation Characteristics of Rice Straw: Analogized with Cd-Impregnated Representative Biomass Components

The pyrolysis characteristics of cadmium (Cd)-impregnated cellulose, hemicellulose, and lignin were studied to elucidate the pyrolysis velocity and deoxygenation characteristics of Cd-contaminated rice straw. The results show that Cd significantly affects the pyrolysis characteristics of a single biomass component. With a heating rate of 5 °C·min−1 and a Cd loading of 5%, the initial pyrolysis temperature of cellulose and hemicellulose decreases while that of lignin increases. The maximum pyrolysis velocity of cellulose, hemicellulose, and lignin is decreased by 36.6%, 12.4%, and 15.2%, respectively. Cd increases the pyrolysis activation energy of the three components and inhibits their deoxygenation. For the pyrolysis of Cd-contaminated rice straw, both the initial depolymerization temperature and the pyrolysis velocity of hemicellulose is reduced, while the pyrolysis velocity of cellulose is accordingly increased. When Cd loading amplifies to 0.1%, 1%, and 5%, the maximum pyrolysis velocity of hemicellulose is decreased by 7.2%, 10.5%, and 21.3%, while that of cellulose is increased by 8.4%, 62.1%, and 97.3%, respectively. Cd reduces the release of volatiles and gas from rice straw, such as CO2, CO, and oxygen-containing organics, which retains more oxygen and components in the solid fraction. This research suggested that Cd retards the pyrolysis velocity and deoxygenation of rice straw, being therefore beneficial to obtaining more biochar.

Manganese-doped ferrihydrite/cellulose/polyvinyl alcohol composite membrane: Easily recyclable adsorbent for simultaneous removal of arsenic and cadmium from soil

Simultaneous removal of arsenic (As) and cadmium (Cd) from soil has been a matter of great concern. In this study, manganese-doped ferrihydrite/cellulose/polyvinyl alcohol composite membranes were prepared via loading manganese-doped ferrihydrite on cellulose mesh and then wrapping with polyvinyl alcohol. The obtained composite membranes exhibited excellent mechanical properties and could be easily separated from soil. The adsorption capacities of Cd(II) and As(III) on the composite membrane were 11.11 mg/g and 72.08 mg/g respectively. After mixing the composite membrane with polluted soil at dosage of 0.3% (w/w), the removal efficiency of the toxicity characteristic leaching procedure leachable As and Cd reached 65% and 69% respectively with one cycle. The stability and reusability of composite membrane were demonstrated by no decrease in tensile strength and the retention of 94% and 96% of As(III) and Cd(II) removal efficiency respectively after four adsorption cycles. The prepared composite membrane had application prospect in the remediation of As and Cd co-polluted soil.

Thermodynamic evaluation on chemical looping conversion of Cd- and Zn-contained phytoremediation plant with different CaO pathways

With the development of phytoremediation for soil contamination, disposal of phytoremediation plant becomes a serious problem. Thermochemical conversion of phytoremediation plant can greatly reduce the volume and mass, meanwhile the clean and reusable utilization is realized. As one of the thermochemical conversion technologies, chemical looping (CL) offers a carbon negative way for clean utilization of biomass. In this technology, CaO has binary roles of heavy metal solidification and CO₂ sorption for gasification enhancement. To assess the CaO pathway in CL of phytoremediation plant, two different CL processes are constructed and comparatively studied based on thermodynamic evaluation. The effects of different operating parameters on the products of gasifier (GR) and reduction reactor (RR) are compared and discussed. Results demonstrated that the CaO addition in GR is beneficial to the production of pure combustible gases. Increasing RR temperature can promote the chemical looping reactions in RR. Under lower temperature, CaO in RR can consume more CO₂ leading to CO₂ free environment. When it is higher than 850 °C, there is no effect of CaO in RR. Increasing the amount of OC in system can enhance the conversion of combustible gases. When α_OC is higher than 0.3, the OC is reduced to a mixed state of Fe₃O₄ and FeO. When the CaO circulates only between GR and calciner, pure CO₂ can be captured at the outlet of calciner. Existence of CaO is beneficial to retain Cd and Zn in solid phases. When the gasification temperature increases from 500 °C to 800 °C, the Cd(g) increases while Cd decreases in both CL1 and CL2. For a long lifetime of OC, CaO is suggested to circulates between GR and calciner.

A review on disposal and utilization of phytoremediation plants containing heavy metals

The reasonable disposal of plant biomass containing heavy metals (HMs) is a difficult problem for the phytoremediation technology. This review summarizes current literature that introduces various disposal and utilization methods (heat treatment, extraction treatment, microbial treatment, compression landfill, and synthesis of nanomaterials) for phytoremediation plants with HMs. The operation process and technical parameters of each disposal method are different. HMs can migrate and transform in different disposal processes. Some disposal and utilization methods can get some by-products. The main purpose of this paper is to provide reference for technical parameters and characteristics of various disposal and utilization methods, so as to choose and use the appropriate method for the treatment of plant biomass containing HMs after phytoremediation.

Properties and heavy metal leaching characteristics of leachate sludge-derived biochar

Heavy metals and metalloids, in sludge and sediments, are environmental pollutants of concern with long-term negative effects on human and ecological health. In this study, sludge from biological treatment of municipal waste leachate was pyrolyzed into leachate sludge-derived biochar (LSDB) at 300°C to 900°C, comprising complex organic and inorganic (particularly heavy metals) species formed from heterogeneous chemical reactions. Based on different advanced material analyses, that is, Thermogravimetric Analysis (TGA), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis, this study revealed that mass loss and microstructural changes of LSDBs occurred primarily due to decomposition of volatiles, aromatic rings, carbonates, and hydroxides. The leaching behaviors of heavy metals from LSDBs were evaluated using the Synthetic Precipitation Leaching Procedure (SPLP). The final pH in SPLP increased from 7.5 to 12.5 with pyrolysis temperature. The pH increase favored the retention of heavy metals in the LSDBs due to the formation of low soluble precipitates at alkaline pH. The heavy metals and metalloids in the LSDBs were present as surface precipitates due to precipitation and cation exchange rather than surface complexation. The leaching contents of metals and metalloids, such as Cr, Cd, Ni, Pb, and As, were all below their respective maximum discharge standards for the first priority pollutants in China.

Brassica Species in Phytoextractions: Real Potentials and Challenges

The genus Brassica is recognized for including species with phytoaccumulation potential and a large amount of research has been carried out in this area under a variety of conditions, from laboratory experiments to field trials, with spiked or naturally contaminated soils, using one- or multi-element contaminated soil, generating various and sometimes contradictory results with limited practical applications. To date, the actual field potential of Brassica species and the feasibility of a complete phytoextraction process have not been fully evaluated. Therefore, the aim of this study was to summarize the results of the experiments that have been performed with a view to analyzing real potentials and limitations. The reduced biomass and low metal mobility in the soil have been addressed by the development of chemically or biologically assisted phytoremediation technologies, the use of soil amendments, and the application of crop management strategies. Certain issues, such as the fate of harvested biomass or the performance of species in multi-metal-contaminated soils, remain to be solved by future research. Potential improvements to current experimental settings include testing species grown to full maturity, using a greater amount of soil in experiments, conducting more trials under real field conditions, developing improved crop management systems, and optimizing solutions for harvested biomass disposal.

Biochar supplementation regulates growth and heavy metal accumulation in tomato grown in contaminated soils

Biochar application has recently gained increased attention to reclaim heavy metal degraded soils. In this context, the present study investigated the effects of biochar on the growth regulation and heavy metal accumulation in tomato grown on contaminated soils. A two-factorial design with factor A including three treatments with mine (contaminated soil) and garden soil in the following ratio viz., T1 = 1:2, T2 = 1:1, and T3 = 2:1, and garden soil only as control; whereas factor B consists of biochar amendments at three levels viz., B1 (3%), B2 (6%), and B3 (9%). Our results revealed significant negative effects of heavy metal-contaminated soil on plant growth, and besides resulted heavy metal accumulation in tomato fruit. Tomato plants showed maximum reduction of growth in T3 followed by T2, and lowest in T1, a similar pattern was found for accumulation of heavy metals in the fruit. However, the application of biochar reduced the bioavailability and accumulation of heavy metals in the tomato fruit, as well as improved plant growth in contaminated soils. Overall, among the three biochar treatments, B2 was determined as the optimum level for improved growth coupled with reduced heavy metal accumulation in the tomato fruit. Besides, biochar application decreased the daily intake of metals and human health risk index values, thus alleviating the health risk. Hence, the present study demonstrated a positive role of biochar in reclaiming heavy metal-contaminated soils and in increasing the plant growth.

The mechanism transformation of ramie biochar's cadmium adsorption by aging

Aging is inevitable when biochar uses for remediate Cadmium (Cd) pollution, but the variation of adsorption mechanism remains unclear. This study uses ramie residue to prepare fresh biochar, and adopt it with acidification and oxidation to simulate the aging process. The difference of physicochemical properties between fresh and aged biochar are studied through microstructure. Then, two kinds of biochar are making adsorption experiments in Cd solution for analyzing their adsorption mechanism. The results show that, both chemisorption and physisorption are exist, chemisorption and physisorption is the predominant way of fresh biochar and aged biochar respectively. Cation exchange is important but weaker in aged biochar than fresh biochar. Carboxyl plays a leading role in complexation of fresh biochar and hydroxy in aged biochar. Coprecipitation and cation-π mechanism impair apparently in aged biochar. This study indicates that aging change ramie biochar's main adsorption mechanism and the primary chemisorption way.

Effect of biochar on the accumulation and distribution of cadmium in tobacco (Yunyan 87) at different developmental stages

Cadmium (Cd) easily accumulates in tobacco, which endangers public health through Cd exposure from smoking. However, its uptake, translocation, and distribution in tobacco plants during plant development or its response to biochar application are poorly understood. A pot experiment was conducted with tobacco (Yunyan 87) grown in soil severely contaminated with Cd (30 mg kg-1) amended with 0, 1, and 2% (w/w) tobacco stem-derived biochar (BC). The absorption and accumulation of Cd in all parts of the tobacco plants were most active from the rosette stage to the fast growing stage, during which approximately 90% of the Cd deposited in the tobacco leaves occurred, especially in the lower leaves. The Cd concentrations in most plant parts without added biochar decreased significantly by 52.61-78.30% due to the rapid increase in biomass (dilution effect), although the Cd concentration in the lower leaves increased by 48.89% (P < 0.05). However, with the slowdown of the growth rate of tobacco at the maturity stage, the proportion of Cd accumulation in roots and stems without biochar addition increased by 29.01%, resulting in an increased Cd concentration in roots and stems by 63.29-86.80% (P < 0.05). In the different growth stages, the application of biochar reduced the contents of DTPA-extractable and exchangeable Cd in the soil by 5.11-35.14% and 9.20-54.05%, respectively, thus reducing the absorption, accumulation and concentration of Cd in all parts of the tobacco plant. In addition, the inhibitive effect of biochar on the Cd concentration in the leaves was weak at the rosette stage (22.17-53.72%) compared with the other stages (46.14-78.88%), and the degree of inhibition of biochar on the Cd concentration in the middle leaves (37.94-59.24%) was lower than that in the upper and lower leaves (49.04-73.54%) at all developmental stages. However, the long-term remediation effect of biochar on soil Cd contamination needs to be further verified, and the combination of biochar and other technologies should receive additional attention.

Utilización de subproductos agroindustriales para la bioadsorción de metales pesados

La contaminación por metales pesados es un problema que afecta a los ambientes acuáticos y terrestres, y cuya principal fuente son las actividades antrópicas. Para atender este problema, la comunidad científica ha desarrollado métodos físico-químicos para la remoción de metales pesados en efluentes contaminados: sin embargo, la mayoría no son económicamente favorables, ya que presentan elevados costos de operación y mantenimiento, además de que algunos generan residuos difíciles de manejar. Sin embargo, existe un método de bajo costo, altamente eficiente y sin formación de contaminantes secundarios, denominado bioadsorción. La bioadsorción utiliza subproductos agroindustriales con el objetivo de utilizar la excesiva generación de estos residuos como bioadsorbentes, para la remoción de metales pesados en aguas residuales. La utilización de subproductos agroindustriales como bioadsorbentes ha mostrado ser una alternativa para su aprovechamiento, consecuentemente, México tiene potencial en la producción de bioadsorbentes. El objetivo de esta revisión es proporcionar información sistematizada del método de remoción de metales pesados por bioadsorción a través del uso de subproductos agroindustriales.

Agronomic Approaches for Characterization, Remediation, and Monitoring of Contaminated Sites

With a view to conserving or improving soil ecosystem services, environment-friendly techniques, such as bio- and phytoremediation, can effectively be used for the characterization, risk assessment, and remediation of contaminated agricultural sites. Polyannual vegetation (meadows, poplar, and cane stands) is widely considered the most efficient tool for remediation (extraction of bioavailable fraction of contaminants), for undertaking safety measures (reducing the mobility of contaminants towards other environmental compartments), and for restoring the ecosystem services of contaminated agricultural sites (biomass production, groundwater protection, C storage, landscape quality improvement, and cultural and educational services). The roles of agronomic approaches will be reviewed by focusing on the various steps in the whole remediation process: (i) detailed environmental characterization; (ii) phytoremediation for reducing risks for the environment and human health; (iii) agronomic management for improving efficiency of phytoremediation; and (iv) biomass recycling in the win-win perspective of the circular economy.