Green synthesis of graphite-based photo-Fenton nanocatalyst from waste tar via a self-reduction and solvent-free strategy.

Thermochemical conversion of biomass yields large quantities of tar as a by-product, which is a potential precursor for the synthesis of renewable carbon-based functional materials. In this study, high-performance photo-Fenton catalyst of graphite­carbon-supported iron nanoparticles was synthesized using a self-reduction and solvent-free approach. The results showed that the tar-derived catalyst had unique properties including a defect-rich graphitic structure, high surface area, and an abundant porous structure resulting from the inherent properties of biomass tar. The iron nanoparticles were highly dispersed within the prepared catalysts and were stably anchored on the carbonaceous surface by the FeC bond. The prepared nanocatalyst showed the highest decomposition constant (91.87 × 10-3 min-1) for 20 mM H2O2, and 40 mg/L RhB can be completely degraded within 2 h under catalyst dosage of 1 g/L and H2O2 addition of 20 mM. The degradation mechanism under the photo-Fenton catalyst/H2O2/light system included the heterogeneous Fenton reaction of iron nanoparticles and photo-Fenton reaction of iron oxide, and the efficient RhB degradation was mainly ascribed to the heterogeneous Fenton reaction. In addition, recycling and leaching tests demonstrated that the photo-Fenton catalyst had excellent reusability and stability, where only 7.3% catalytic reactivity was reduced after five cycles. This work provided a green, sustainable, and facile approach for synthesizing photo-Fenton catalysts by value-added utilization of tar wastes.

Efficient immobilization of toxic heavy metals in multi-contaminated agricultural soils by amino-functionalized hydrochar: Performance, plant responses and immobilization mechanisms.

A novel amino-functionalized hydrochar material (referred to NH2-HCs) was prepared and used as the soil amendment to immobilize multi-contaminated soils for the first time. The results showed that the application of NH2-HCs significantly improved (P < 0.05) soil properties (i.e., pH value, cation exchange capacity and organic content). By introduction of NH2-HCs, the contaminated soil showed the highest value of 96.2%, 52.2% and 15.5% reductions in Cu, Pb and Cd bioavailable concentrations and the leaching toxicity of Cu, Pb and Cd were remarkably reduced by 98.1%, 31.3% and 30.4%, respectively. Most of exchangeable Cu, Pb and Cd reduced were transformed into its less available forms of oxidizable and residual fractions. Potential ecological risk assessment indicated that the element Cd accounted for the most of total risks in NH2-HCs amended soils. The mechanism study indicated that surface complexation, chemical chelating and cation-pi interaction of NH2-HCs played a vital role in the immobilization of heavy metals. Pot experiments further verified that the application of NH2-HCs significantly improved plant growth and reduced metal accumulations. The present study offered a novel approach to prepare amino-functionalized hydrochars with great potential as the green and alternative amendments for efficiently immobilizing heavy metals in multi-contaminated soil.

Synthesis of biomass tar-derived foams through spontaneous foaming for ultra-efficient herbicide removal from aqueous solution.

Pyrolysis is one of the most important approaches to convert waste biomass into renewable energy and biomaterials, and the tar is the inevitable by-product of this process. In this study, carbon foams were prepared innovatively with biomass tar as the precursor through spontaneous gas foaming approach and used for dicamba removal from aqueous solution. The results showed that prepared carbon foams had unique properties including rich microporous structure and high specific surface area (reaching 1667 m2/g). In addition, the prepared carbons had high thermal stability due to the high graphitic degree. The adsorption results indicated that pH showed a great effect on the adsorption of dicamba onto the prepared carbon foams. The carbon foam exhibited ultra-fast dicamba removal and ultra-high adsorption capacity of 891.74 mg/g at room temperature. The adsorption process was well described by pseudo-second-order kinetics and Langmuir isotherm models. The thermodynamic study indicated dicamba adsorption onto the prepared carbon foams was a spontaneous and exothermic process. In addition, the good reusability from recovery test demonstrated that the prepared carbon foams had promising potential for dicamba removal from aqueous solution.

Pyrolysis kinetics and thermodynamic parameters of the hydrochars derived from co-hydrothermal carbonization of sawdust and sewage sludge using thermogravimetric analysis.

The thermal behavior of the hydrochars from co-hydrothermal carbonization (co-HTC) of sawdust (SD) and sewage sludge (SS) was investigated using thermogravimetric analysis. The comprehensive devolatilization index indicated that the devolatilization performance of SS was decreased by HTC, while it was significantly improved 7.38-23.69 times by co-HTC. The kinetic analysis showed that HTC of SS decreased the average activation energy from 308.96 and 314.78 kJ mol-1 to 220.86 and 221.27 kJ mol-1 by Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), respectively, while it was increased from 182.37 to 207.06 kJ mol-1 and from 181.06 to 207.05 kJ mol-1 with the increasing proportion of SD from 25% to 75% during co-HTC, respectively. The thermodynamic parameters revealed that pyrolysis reactivity of the hydrochar derived from SD was improved by co-HTC of SD and SS. Kinetic and thermodynamic findings were useful for the design of pyrolysis process using hydrochar as solid fuel.

Effect of hydrothermal carbonization on heavy metals in swine manure: Speciation, bioavailability and environmental risk.

The speciation, bioavailability and environmental risk of heavy metals (HMs) in the hydrochar derived from hydrothermal carbonization (HTC) of swine manure were investigated in this study. The results indicated that the majority of HMs originally in swine manure were retained in the hydrochar by HTC, and CaO addition substantially reduced the HMs concentration in the hydrochar. HTC especially CaO assisted HTC significantly promoted the HMs transformation from the bioavailable fraction to the relatively stable fraction, and thus decreased their environmental risk in the hydrochar. Moreover, the Toxicity Characteristic Leaching Procedure and diethylenetriamine pentaacetic acid test revealed that the leachability and plant-bioavailability of HMs in swine manure were greatly declined by HTC especially for HTC with 15% CaO addition. The present study suggested that HTC was an effective disposal approach for swine manure from the perspective of HMs immobilization, especially for CaO assisted HTC.

Co-hydrothermal carbonization of corn stalk and swine manure: Combustion behavior of hydrochar by thermogravimetric analysis.

The combustion behavior of the hydrochar from co-hydrothermal carbonization (HTC) of corn stalk (CS) and swine manure (SM) was thermogravimetrically investigated. Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) were used for kinetic analysis, and the thermodynamic parameters were determined. Results showed that HTC decreased the combustion property and stability of SM, while co-HTC with CS significantly improved the combustion performance of the hydrochar including the increased ignition temperature and decreased burnout temperature. HTC of SM decreased the average activation energy (Ea) value from 140.40 and 137.31 KJ/mol to 124.40 and 120.17 KJ/mol by FWO and KAS, respectively, and increasing proportion of CS during co-HTC increased the Ea value of the hydrochar to 141.53-171.23 and 138.35-169.66 KJ/mol, respectively. The thermodynamic parameters confirmed the enhanced combustion reactivity of the hydrochar from co-HTC of CS and SM. These findings demonstrated that co-HTC with CS benefited the hydrochar production from SM with improved combustion performance.

Properties of hydrochars derived from swine manure by CaO assisted hydrothermal carbonization.

The hydrochars derived from swine manure were prepared by CaO assisted hydrothermal carbonization (HTC), and their properties were investigated for the first time. The results showed that the pH and yield of the hydrochar were largely increased by CaO addition. HTC of swine manure increased the phosphorus (P) content in the hydrochar, and appropriately 100% of P as apatite-P was enriched in the hydrochar by CaO assisted HTC. Additionally, the CaO addition during HTC improved the porosity of the hydrochar. The FTIR analysis revealed that substantial functional groups were present on the surface of the hydrochar, indicating the facilitated exchange between the hydrochar and hydrophilic soil when the hydrochar was used for soil amendment. This study demonstrated that CaO assisted HTC was a novel strategy to quickly convert swine manure to the promising soil amendment especially for acidic soils.

Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: Hydrochar properties and heavy metal transformation behavior.

Co-hydrothermal carbonization (HTC) of lignocellulosic biomass and swine manure (SM) was conducted, and the hydrochar properties and transformation behavior of heavy metals (HMs) were investigated in this study. The results showed that co-HTC with lignocellulosic biomass promoted the dehydration of SM and enhanced the aromatization of the hydrochar. Compared to the hydrochar from SM, the carbon content, higher heating value and energy yield of the hydrochar from co-HTC were significantly increased, reaching the maximum of 57.05%, 24.20 kJ/kg and 80.17%, respectively. Significant synergy occurred between lignocellulosic biomass and SM during co-HTC and different lignocellulosic biomass exhibited similar influence on the synergy. Additionally, the concentration and bioavailability of HMs in the hydrochar from co-HTC were decreased in comparison to SM. These findings suggested that co-HTC with lignocellulosic biomass offered an effective approach to convert SM into clean solid fuel with remarkably improved fuel properties.

Effect of hydrothermal carbonization on migration and environmental risk of heavy metals in sewage sludge during pyrolysis.

The heavy metals distribution during hydrothermal carbonization (HTC) of sewage sludge, and pyrolysis of the resultant hydrochar was investigated and compared with raw sludge pyrolysis. The results showed that HTC reduced exchangeable/acid-soluble and reducible fraction of heavy metals and lowered the potential risk of heavy metals in sewage sludge. The pyrolysis favored the transformation of extracted/mobile fraction of heavy metals to residual form especially at high temperature, immobilizing heavy metals in the chars. Compared to the chars from raw sludge pyrolysis, the chars derived from hydrochar pyrolysis was more alkaline and had lower risk and less leachable heavy metals, indicating that pyrolysis imposed more positive effect on immobilization of heavy metals for the hydrochar than for sewage sludge. The present study demonstrated that HTC is a promising pretreatment prior to pyrolysis from the perspective of immobilization of heavy metals in sewage sludge.

Heteroatoms doped porous carbon derived from hydrothermally treated sewage sludge: Structural characterization and environmental application.

The heteroatoms (N and S) doped porous carbons (HAPCs) were prepared from sewage sludge by hydrothermal carbonization and chemical activation for the first time. The porous structures and surface properties of HAPCs were characterized by multiple techniques including SEM-EDS, TEM, BET, XRD, Raman spectroscopy and Boehm's titration. The resultant materials were showed to be naturally N and S dual-doped porous carbons (HAPCs), especially for HAPCHCl+HF obtained by HCl-HF-washing, which was typical 3D hierarchically porous structure with abundant mesopores as well as big pore diameter. Then the performance of HAPCHCl+HF on AO7 removal was determined through Response surface methodology. The results showed the adsorption behavior obeyed Langmuir isotherm model and the maximum adsorption capacity was up to 440.53 mg g-1 at 25 °C. Kinetics study revealed that the adsorption followed pseudo second-order kinetic and intra-particle diffusion was the main control step. The high removal rate of AO7 was ascribed to the unique properties of HAPCHCl+HF. The great Vmes and big pore diameter facilitated the diffusion of AO7 into the intra surface of particle. Meanwhile, the basic groups and doping of N and S made HAPCHCl+HF surface had positive charges, then strong π-π stacking interaction and electrostatic attraction contributed to the highly effective adsorption. This study indicated hydrothermal carbonization coupled with chemical activation was a cost-effective approach to prepare efficient heteroatoms doped porous carbon from sewage sludge towards azo dye contaminated wastewater treatment.

Removal of azo dye by a highly graphitized and heteroatom doped carbon derived from fish waste: Adsorption equilibrium and kinetics.

A highly graphitized and heteroatom doped porous carbon was prepared from fish waste in the present study. The morphology and chemical composition of the resultant porous carbon were characterized by SEM-EDS, TEM, BET, XRD and Raman measurement. The prepared porous carbon was employed as an adsorbent for acid orange 7, a typical azo dye, removal from aqueous solution. The results showed that the porous carbon had ultrahigh surface area of 2146 m(2)/g, a high degree of graphitization structure and naturally doped with nitrogen and phosphorous. The maximum adsorption capacity of acid orange 7 reached 285.71 mg/g due to unique property of the prepared porous carbon. In addition, acid orange 7 adsorption onto the porous carbon well followed pseudo-second-order kinetics model and acid orange 7 diffusion in micropores was the potential rate controlling step.

Emission, distribution and toxicity of polycyclic aromatic hydrocarbons (PAHs) during municipal solid waste (MSW) and coal co-combustion.

Emission and distribution characteristics of polycyclic aromatic hydrocarbons (PAHs) were investigated during municipal solid waste (MSW) and coal combustion alone and MSW/coal blend (MSW weight fraction of 25%) co-combustion within a temperature range of 500°C-900°C. The results showed that for all combustion experiments, flue gas occupied the highest proportion of total PAHs and fly ash contained more high-ring PAHs. Moreover, the 3- and 4-ring PAHs accounted for the majority of total PAHs and Ant or Phe had the highest concentrations. Compared to coal, MSW combustion generated high levels of total PAHs with the range of 111.28µg/g-10,047.22µg/g and had high toxicity equivalent value (TEQ). MSW/coal co-combustion generated the smallest amounts of total PAHs and had the lowest TEQ than MSW and coal combustion alone. Significant synergistic interactions occurred between MSW and coal during co-combustion and the interactions suppressed the formation of PAHs, especially hazardous high-ring PAHs and decreased the TEQ. The present study indicated that the reduction of the yield and toxicity of PAHs can be achieved by co-combustion of MSW and coal.

Co-liquefaction of microalgae and lignocellulosic biomass in subcritical water.

This study investigated co-liquefaction of microalgae (Chlorella pyrenoidosa, CP) and lignocellulosic biomass (Rice husk, RH) in subcritical water for bio-oil production. The effects of liquefaction temperature (200-350°C), residence time (10-90min), solid concentration (10-30wt.%) and mass ratio of CP/RH on product distribution were investigated. The results showed that the highest yield of bio-crude oils at the combination of 50% CP with 50% RH was obtained at 300°C temperature, 60min residence time and 20wt.% solid concentration. The oil yields increased gradually with the increased mass ratio of CP/RH. The major compounds identified in bio-crude oils from hydrothermal liquefaction (HTL) of RH were cyclic oxygenates (20.62%), followed by esters, ketones and alcohols (17.19%). As for CP, the main components were straight & branched amides (28.38%). A synergistic interaction was observed between CP and RH during co-liquefaction, resulting in decreased acidity and nitrogen content of bio-crude oils.

Combustion behavior and kinetics of low-lipid microalgae via thermogravimetric analysis.

Thermogravimetric analysis and differential thermal analysis were employed to investigate combustion characteristics of two low-lipid microalgae, Chlorella pyrenoidosa (CP) and Spirulina platensis (SP) and iso-conversional Starink approach was used to calculate the kinetic parameters in the present study. The results showed that three stages of mass loss, including dehydration, devolatilization and char oxidation, were observed during combustion of both of two low-lipid microalgae. The whole weight loss of combustion of two microalgae was both shifted to higher temperature zones with increased heating rates from 10 to 40 K/min. In the 0.1-0.9 conversion range, the apparent activation energy of CP increased first from 51.96 to 79.53 kJ/mol, then decreased to 55.59 kJ/mol. Finally, it slightly increased to 67.27 kJ/mol. In the case of SP, the apparent activation energy gradually increased from 68.51 to 91.06 kJ/mol.

Characterization of aqueous phase from the hydrothermal liquefaction of Chlorella pyrenoidosa.

This study investigated the characteristics of aqueous phase from hydrothermal liquefaction of low-lipid microalgae Chlorella pyrenoidosa. The interactions of operating conditions, including reaction temperature, retention time and total solid ratio were evaluated by response surface methodology. The chemical oxygen demand, total nitrogen and total phosphorus were selected as indicators of the property of AP. Results indicated that total solid ratio was found to be the dominant factor affecting the nutrient recovery efficiencies of AP. Based on energy recovery, GC-MS indicated that the AP at two optimized operating conditions (280 °C, 60 min, 35 wt.% and 300 °C, 60 min, 25 wt.%) were observed to have a higher concentration of organic acids (10.35% and 8.34%) while the sample (260 °C, 30 min, 35 wt.%) was observed to have the highest concentration of N&O-heterocyclic compounds (36.16%).

Physical pretreatments of wastewater algae to reduce ash content and improve thermal decomposition characteristics.

Previous study showed high ash content in wastewater algae (WA) has a negative effect on bio-crude oil formation in hydrothermal liquefaction (HTL). This study explored the effect of different pretreatments on ash reduction and the thermal decomposition of WA. Single-stage (e.g. centrifugation) and two-stage pretreatments (e.g. centrifugation followed by ultrasonication, C+U) were used. The apparent activation energy of the thermal decomposition (E(a)) of pretreated algae was determined. HTL was conducted to study how different pretreatments may impact on bio-crude oil formation. Compared to untreated samples, the ash content of algae with centrifugation was reduced from 28.6% to 18.6%. With C+U pretreatments, E(a) was decreased from 50.2 kJ/mol to 35.9 kJ/mol and the bio-crude oil yield was increased from 30% to 55%. These results demonstrate that pretreatments of C+U can improve the thermal decomposition behavior of WA and enhance the bio-crude oil conversion efficiency.

Thermogravimetric and kinetic analysis of thermal decomposition characteristics of low-lipid microalgae.

The thermal decomposition behavior of two microalgae, Chlorella pyrenoidosa (CP) and Spirulina platensis (SP), were investigated on a thermogravimetric analyzer under non-isothermal conditions. Iso-conversional Vyazovkin approach was used to calculate the kinetic parameters, and the universal integral method was applied to evaluate the most probable mechanisms for thermal degradation of the two feedstocks. The differential equations deduced from the models were compared with experimental data. For the range of conversion fraction investigated (20-80%), the thermal decomposition process of CP could be described by the reaction order model (F3), which can be calculated by the integral equation of G(α) = [(1 - α)(-2) - 1]/2. And the apparent activation energy was in the range of 58.85-114.5 kJ/mol. As for SP, it can be described by the reaction order model (F2), which can be calculated by the integral equation of G(α) = (1 - α)(-1) - 1, and the range of apparent activation energy was 74.35-140.1 kJ/mol.

The kinetic analysis of the pyrolysis of agricultural residue under non-isothermal conditions.

The study concerns the pyrolysis kinetics of agricultural wastes, corn straw (CS) and rice husk (RH). Thermogravimetric experiments were carried out in a thermogravimetric analyzer under inert conditions, and operated at different heating rates ranging from 5 to 40K/min. As the increment of heating rates, the variations of characteristic parameters from the TG-DTG curves were determined. Iso-conversional Starink approach and Avrami theory were used to evaluate the kinetic parameters, including apparent activation energy and reaction order. For the range of conversion fraction investigated (20-80%), the apparent activation energy of CS initially increased from 98.715 to 148.062 kJ/mol and then decreased to 144.387 kJ/mol afterwards, whilst the apparent activation energy of RH increased gradually from 50.492 to 88.994 kJ/mol. With varied temperatures (517-697 K), the corresponding value of reaction order was increased from 0.288 and 0.359 to 0.441 and 0.692, along with a decrease to 0.306 and 0.445, respectively.