Preparation of low-cost sludge-based highly porous biochar for efficient removal of refractory pollutants from agrochemical and pharmaceutical wastewater.

Producing a high-performance sludge biochar through a feasible method is a great challenge and is crucial for practicability. Herein, we reported a highly porous sludge biochar synthesized from agrochemical-pharmaceutical and municipal sludge blends through a novel pyrolysis-acid treatment-post pyrolysis method. The optimized biochar named ASMS91 obtained interconnected pores with a total pore volume of 0.894 cm3/g and a surface area of 691.4 m2/g through extended acid wash and subsequent post-pyrolysis, which is superior to non-activated sludge biochar. ASMS91 removed 45.3 % of wastewater COD (156 mg/L) in 24 h, which was rapid and higher performance than commercial activated carbon (1000 iodine number). This outstanding performance is due to its high adsorption ability of long-chain aliphatic compounds (e.g., 2,4-Di-tert-butylphenol, neophytadiene and eicosane) into mesopores, which accounts for 71.8 % of pore filling. ASMS91 was highly recyclable, and adsorption was reduced by only 5.3 % after the 4th cycle. It also outperformed other sludge biochar in literature in removing perfluorooctanoic acid (PFOA), 6:2 fluorotelomer sulfonate (6:2 FTS), sulfamethoxazole, methylene blue, and methylene orange. Finally, the feasibility of our proposed method was validated by a brief techno-economic analysis. This feasible approach may support future research regarding sludge valorization and low-cost chemical wastewater treatment.

Catalytic degradation of crystal violet and methyl orange in heterogeneous Fenton-like processes.

Metals-loaded (Fe3+, Cu2+ and Zn2+) activated carbons (M@AC) with different loading ratios (0.1%, 0.5%, 1%, 5% and 10%) were prepared and employed for catalytic degradation of dye model compounds (crystal violet (CV) and methyl orange (MO)) in wastewater by heterogeneous Fenton-like technique. Compared with Cu@AC and Zn@AC, 0.5% Fe3+ loaded AC (0.5Fe@AC) had better catalytic activity for dyes degradation. The effects of dyes initial concentration, catalyst dosage, pH and hydrogen peroxide (H2O2) volume on the catalytic degradation process were investigated. Cyclic performance, stability of 0.5Fe@AC and iron leaching were explored. Degradation kinetics were well fitted to the pseudo-second-order model (Langmuir-Hinshelwood). Almost complete decolorization (99.7%) of 400 mg L-1 CV was achieved after 30 min reaction under the conditions of CV volume (30 mL), catalyst dosage (0.05 g), H2O2 volume (1 mL) and pH (7.7). Decolorization of MO reached 98.2% under the same conditions. The abilities of pyrolysis char (PC) of dyeing sludge (DS) and metal loaded carbon to remove dye pollutants were compared. The intermediate products were analyzed and the possible degradation pathway was proposed. This study provided an insight into catalytic degradation of triphenylmethane- and aromatic azo-based substances, and utilization of sludge char.

TiO2/activated carbon synthesized by microwave-assisted heating for tetracycline photodegradation.

A furfural residue-derived activated carbon (AC) supported black-TiO2 photocatalyst was successfully prepared by ultrasonic-assisted sol-gel treatment (USG) and solvothermal treatment (ST) combined with microwave-assisted heating (MH). The prepared composites were characterized and evaluated based on the degradation of tetracycline hydrochloride (TC) under ultraviolet (UV) illumination. The average TiO2 nanoparticle size of the as-synthesized catalysts was between 9 and 11 nm. The bandgap of TiO2-USGM was 1.6 eV, much lower than that of other reference catalysts. Organic carbon and AC in the catalyst play positive roles in reducing the band gap (e.g. 1.6∼2.6 eV) and improving visible-light absorption. The oxygen vacancies are responsible for UV-visible absorption. Adding AC into black TiO2 resulted in a lower degree of recombination of photogenerated electrons. Mott-Schottky plots showed that AC-containing TiO2@AC-STM reduced the value of conduction band value from -0.59 eV to -0.24 eV, which is beneficial to photogenerated electrons. Compared with TiO2, the Ti-O-C and Ti-C- in TiO2@AC remarkably improved the adsorption and catalytic efficiency of TC. In a near-neutral pH environment, TiO2@AC-STM and TiO2@AC-USGM exhibited high removal efficiencies (88.0% and 75.7%, respectively) and degradation rates (0.0418 and 0.0302 µmol/g/s, respectively) at a catalyst load of 0.25 g/L. Notably, the catalyst can be effectively used over a wide range of pH (6-9). The solution pH after treatment was close to neutral, which is advantageous for wastewater treatment. The activation energies were found to be approximately 3.47 kJ/mol. The thermodynamic parameters showed that the photodegradation process was non-spontaneous and endothermic. Based on the trapping experiments, O2⋅- was mainly responsible for TC photodegradation over TiO2@AC-STM, followed by h+. The TC degradation pathways and catalyst stability were also investigated. Biomass-derived carbon-supported catalysts have great potential for waste biomass utilization as green, and low-cost catalysts.

Iron-biochar production from oily sludge pyrolysis and its application for organic dyes removal.

In this study, a single-step pyrolysis approach was developed to directly convert oily sludge (OS) with high iron content into a magnetic iron-char catalyst for organic dyes removal. Magnetic iron-char catalysts were employed to degrade crystal violet (CV), methylene blue (MB), and sunset yellow (SY). The OC800 iron-char catalyst prepared from OS was not only rich in iron (mainly stable Fe3O4), but also showed favorable pore structures. Effects of operation parameters like temperature, H2O2 dosage, and pH on dye removal based on Fenton degradation were examined. In OC800 Fenton system (0.5 mL H2O2, 500 mg/L dye concentration, and pH = 2 in 50 mL solution), the maximum dye removal capacities of SY, CV, and MB were 83.61, 639.19, and 414.25 mg/g, respectively. In dyes degradation experiments, the prepared catalyst could be reused (more than 3 successive cycles) due to higher stability and less leaching of iron. One-step pyrolysis of OS with high iron content thereby represents a promising approach to transform sludge waste to functional biochar that removes hazardous dyes.

Microwave pyrolysis of oily sludge under different control modes.

The effects of temperature and power on product distribution and characteristics of oily sludge (OS) pyrolysis were investigated in a microwave reactor. The maximum oil yield was 72.55 wt% at 550 °C and 71.47 wt% at 800 W, respectively. X-ray photoelectron spectroscopy (XPS) indicated that C-C and C-O were the main forms of carbon in OS char (OC). The sulfur (S) content in OC increased as the temperature/power rose, implying that S might exist in the form of inorganics or OC had S retention ability. In temperature control mode, the changes of functional groups on OC surface were more sensitive. The maximum hydrocarbon content in oil was 14.56% at 350 °C and 13.40% at 900 W, respectively. The contents of oxygenated compounds and heterocycles in oil from temperature control mode were higher. The CO yield increased with increasing temperature/power, reaching the maximum of 9.60 wt% at 650 °C and 7.75 wt% at 900 W, respectively. Compared with power control mode, it seemed that more heavy metals (HMs) were retained in OC in temperature control mode. The Er of HMs were at the clean level and RI indicated the HMs in OC had a low environmental risk.

Microwave-assisted pyrolysis of oily sludge from offshore oilfield for recovery of high-quality products.

Microwave pyrolysis of oily sludge (OS) was investigated in this study. In this case, the highest oil yield (85.93 wt%) was achieved at 500 °C. The molar ratio of H/C was lower for OS char (OC) at higher pyrolysis temperatures, indicating good stability of OC owing to high degree of carbonization and aromaticity. Then, iodine adsorption value of OC reached maximum (531.2 mg/g) at 750 °C. While methylene blue (MB) uptake slightly increased with temperature and reached maximum (384.08 mg/g) at 850 °C. In order to improve the quality of pyrolysis products, different catalysts were employed in OS pyrolysis. The maximum content (64.31%) of aromatic hydrocarbon was found in PO500-10ß. In addition, ß-zeolite also reduced oxygenates content in oil, beneficial for stability of oil products. The gas products from catalytic pyrolysis were abundant in CO and CH4, and KOH achieved the highest CO (5.9 wt%), CH4 (16.9 wt%) and H2 (2.4 wt%) yields. Finally, a reaction mechanism pathway for OS pyrolysis was proposed to show the production routes of gas, liquid, and solid products.

Petroleum oil and products recovery from oily sludge: Characterization and analysis of pyrolysis products.

Oily sludge (OS) has attracted special interest because of its hazardous nature and high potential as an energy resource. This study investigated the oil recovery from OS by thermal cracking and catalytic pyrolysis. The oil yield increased when the temperature exceeded 450 °C and reached a maximum (76.84 wt%) at 750 °C. Catalysts significantly improved the quality of oil produced during catalytic pyrolysis. Aromatic hydrocarbons were dominant (10.01-52.69%) in pyrolysis oil (PO) from OS catalytic pyrolysis, and the catalysts significantly reduced the presence of oxygen heterocycles. In addition, KOH and CaO reduced the ID (D-band peak intensity)/IG (G-band peak intensity) of OS char (OC) and increased the degree of graphitization. Owing to its higher iodine adsorption value and methylene blue (MB) adsorption value, OC exhibits potential as an adsorbent. The environmental assessment and potential applications of OC, along with possible reaction mechanisms and kinetic characteristics, are also discussed.

Characterization and analysis of condensates and non-condensable gases from furfural residue via fast pyrolysis in a bubbling fluidized bed reactor.

Pyrolysis of furfural residue (FR) was performed at 450-850 °C by employing a fluidized bed pyrolyzer (FBP). Addition of Kaolin and Ca-bentonite to FR considerably increased the condensate yields. The highest condensate yield (24.96 wt%) was obtained at 650 °C when Ca-bentonite was added. Fourier transform infra-red (FTIR) spectrum of pyrolysis oil (PO) indicated that catalysts promoted generation of alkene, amine, sulfate, sulfonyl chloride and oxime during pyrolysis. Gas chromatography mass spectrometry (GC-MS) demonstrated that catalysts significantly increased the content of furfural and phenol in PO and the maximum phenol content (15.36%) was achieved in PO650-3 for CaO. The quite low relative proportion (RP) of ammonia nitrogen in liquid indicated that the dominant form of nitrogen in liquid was not ammonia nitrogen. CaO had the ability to reduce H2S release, indicating significant sulfur retention capacity. The maximum RP (99.29%) of chlorine in bio-char (BC) was observed with the addition of CaO, showing its strong chlorine retention capacity.

Microwave-assisted catalytic pyrolysis of waste printed circuit boards, and migration and distribution of bromine.

Microwave-assisted pyrolysis (MAP) of waste printed circuit boards (WPCB) was performed to investigate the characteristics of pyrolysis product and Br fixation. Pyrolysis conversion increased with increasing temperature, reaching 93.3 % at 650 °C. However, increasing heating time did not exhibit remarkable influence on pyrolysis conversion. At 350 °C, phenols were main compounds in the oil accounting for 91.15 %. As the temperature increased to 650 °C, polycyclic aromatic hydrocarbons and monocyclic aromatic hydrocarbons (except phenols) increased to 20.55 % and 19.03 %, respectively. Meanwhile, the total content of CO2, CO, CH4 and H2 in the non-condensable gases increased significantly. Addition of ZSM-5 and kaolin promoted the recombination reaction of pyrolysis products, increased the relative percentage of monocyclic aromatic hydrocarbons (except phenols) and C11-C20 compounds in the oil, and reduced non-condensable gases. The oxygen bomb-ion chromatography was used to evaluate the Br content of pyrolysis residues. Higher pyrolysis temperature enhanced transfer of Br to pyrolysis gas. K2CO3, Na2CO3 and NaOH reacted with hydrogen bromide to generate KBr and NaBr, which significantly improved the Br fixation efficiency of pyrolysis residues (i.e. from 29.11%-99.80%, 96.39 % and 86.69 %, respectively) and reduced Br content in pyrolysis gas.

Torrefaction subsequent to pelletization: Characterization and analysis of furfural residue and sawdust pellets.

Torrefaction integrated with pelletization has gained increasingly interest as it enhances the characteristics of fuel pellets (e.g. hydrophobicity and energy density). In current study, torrefaction of furfural residue pellets (FRPs) and sawdust pellets (SPs) was performed by employing tubular reactor furnace, and quality of pellets was compared. The characteristics of both types of pellets were significantly improved with increasing torrefaction temperature from 200 °C to 300 °C and residence time from 15 min to 30 min. The highest lower heating value of 23.78 MJ/kg and energy density ratio (1.27) for torrefied furfural residue pellets (TFRPs) and 26.76 MJ/kg and 1.46 for torrefied sawdust pellets (TSPs) were achieved at 300 °C and 120 min. Increasing torrefaction temperature and residence time, the volumetric energy densities of TFRPs increased from 25.69 (at 200 °C and 15 min) to 27.59 kJ/m3 (at 300 °C and 120 min), while those of TSPs correspondingly decreased from 20.81 to 16.69 kJ/m3. The highest true densities (i.e. 2.40 and 1.85 g/cm3) and porosities (i.e. 52 and 65 v %) of TFRPs and TSPs were achieved at 300 °C and 120 min, much higher than those of un-torrefied pellets. Moisture uptake of TFRPs and TSPs at 300 °C were only 1.4 wt% and 2.0-2.8 wt%, respectively, showing strong water-resistant ability. The crystallinity of cellulose in FRPs was found higher than that of SPs, while the crystallinity of cellulose in TFRPs was found lower than that of TSPs at same process conditions. FTIR showed that O-H bond was destroyed after torrefaction for both FRP and SP.

Comparative study for fluidized bed pyrolysis of textile dyeing sludge and municipal sewage sludge.

A comparative research was performed to evaluate the products yields and chars properties for pyrolysis of textile dyeing sludge (TDS) and municipal sewage sludge (MSS). The high fixed carbon (19.36 wt%) and low volatile (23.66 wt%) contents of TDS resulted in higher char yields and lower condensate yields. TDS char (TC) had a higher sulfur (S) retention efficiency than MSS char (MC) and CaO exhibited a great S retention effect in MC. More alkali and alkaline earth metals (e.g. Na, K, Mg and Ca) in MSS contributed to enhanced catalytic pyrolysis. In comparison to non-catalytic pyrolysis, chars from catalytic pyrolysis had lower iodine number and higher methylene blue (MB) adsorption value. MB adsorption values of MC (212.28-414.20 mg/g) were much higher than those of TC (84.32-156.07 mg/g). In contrast, heavy metals risk degrees of MC (4.20-7.56) were lower than those of TC (7.55-12.87), and heavy metals in TC and MC showed slight risks to environment.

Microwave-assisted pyrolysis of textile dyeing sludge, and migration and distribution of heavy metals.

This study investigated the pyrolysis characteristic of textile dyeing sludge (DS) using an auger pyrolyser under microwave irradiation at different pyrolysis temperature. The migration and distribution characteristic of heavy metals and their potential ecological risks were investigated using inductively coupled plasma mass spectrometry (ICP-MS) techniques. Temperature and additives (e.g. Ca-bentonite, kaolin and CaO) significantly affected product distribution and yields. Heavy metals showed different enrichment characteristics during pyrolysis and a great percentage of the heavy metals was retained in the sludge char (SC), depending on different temperatures and additives. CaO had a strong ability for retention of S, Pb and Ni. Ca-bentonite, kaolin and CaO had considerable ability to retain Cr at 650 ℃. Ca-bentonite and CaO had positive effects on Ni retention at 450 °C and 650 °C. As was enriched at 450 °C with addition of Ca-bentonite. Cu and Zn were enriched at 450 °C and 850 °C without additives and the corresponding residue ratios (RRs) were 88.68-100%, which indicated good stability of these heavy metals during microwave pyrolysis of DS. The heavy metal contents in SC were lower than those in the national standards (CJ/T 362-2011, China) and these heavy metals showed slight potential ecological risk to the environment.

Microwave pyrolysis of textile dyeing sludge in a continuously operated auger reactor: Condensates and non-condensable gases.

This paper investigated an auger pyrolyser under microwave irradiation using textile dyeing sludge (DS) as the feedstock. Microwave power, temperature, auger speed, gas velocity and addition of catalysts were studied. In terms of ICP-MS, Cu and As concentrations in condensates, depending on pyrolysis temperatures, exceeded the wastewater discharge standard in China. The condensate and oil yields reached maximum (i.e. 12.86 wt% and 0.84 wt%, respectively) at 650 °C. The content of aromatic compounds in the oil increased as temperature increased, up to 88.38% (GC-MS area) at 750 °C. Heterocyclic aromatic compounds containing nitrogen accounted for 20%-58% of the pyrolysis oil. Addition of catalysts such as CaO and Fe decreased pyrolysis oil yield and increased the content of H2. The H2 content increased from 25.39v% without catalyst to 64.17v% with addition of 30 wt% CaO. The electricity consumption was 0.80-2.64 kWh/kg wet sludge from 450 to 750 °C and auger speed range of 1-9 rpm. Higher auger speeds and lower temperatures led to lower electricity consumption.

Microwave pyrolysis of textile dyeing sludge in a continuously operated auger reactor: Char characterization and analysis.

Textile dyeing sludge (DS) contains toxic organic and inorganic substances, and needs to be disposed by appropriate technologies. This paper investigated an auger pyrolyser under microwave irradiation for treatment and disposal of DS. Microwave power, temperature, gas and solid residence times, addition of catalysts were studied. The results showed that the char yield gradually decreased with microwave power. The residence times of solid and gas have slight effects on char yields in the temperature range of 450-750°C. Addition of CaO and Fe increased char yield, decreased non-condensable gas yield and increased H2 content. CaO promoted decomposition of DS and capture of CO2. In the meantime, CaO retained more sulfur in the solid phase compared to MWP without addition of CaO. The content of C, H and N in DS and sludge char (SC) decreased with temperature while S increased. The maximum BET (Brunauer-Emmett-Teller) surface area of 91.9m2/g was observed for SC at 550°C, consistent with Scanning Electron Microscopy (SEM) observations. X-Ray fluorescence (XRF) analysis showed existence of some heavy metals, and leaching characteristics of heavy metals in the SC meet the requirements of national standards in China and will not pose a threat to the environments.