Yunbo Zhai

Influence of sewage sludge-based activated carbon and temperature on the liquefaction of sewage sludge: Yield and composition of bio-oil, immobilization and risk assessment of heavy metals

The influence of sewage sludge-based activated carbons (SSAC) on sewage sludge liquefaction has been carried out at 350 and 400°C. SSAC increased the yield and energy density of bio-oil at 350°C. The metallic compounds were the catalytic factor of SSAC obtained at 550°C (SSAC-550), while carbon was the catalytic factor of SSAC obtained at 650°C. Liquefaction with SSAC redistributed the species of heavy metals in solid residue (SR). With the addition of SSAC, the risk of Cu, Zn and Pb decreased at 350°C, while at 400°C the risk of Cd, Cu, and Zn were decreased. Ecological risk index indicated that 400°C was preferable for the toxicity decrement of SR, while risk assessment code indicated that SR obtained at 350°C contained lower risk. Considering the bio-oil yield, liquefaction at 350°C with SSAC-550 was preferable.

The fate of Cu, Zn, Pb and Cd during the pyrolysis of sewage sludge at different temperatures

In the present study, a sequential extraction procedure, recommended by the Community Bureau of Reference (BCR), was used for the fractionation of the heavy metals Cu, Zn, Pb and Cd in sewage sludge and its residues produced after pyrolysis at different temperatures from 250 to 700 °C. The results show that, in the sludge sample, the sum of the percentages of the reducible and oxidizable fractions for all metals except Cu was very high (65.4% for Cd, 85.7% for Pb, 78.7% for Zn), whereas the sum of the percentages of the oxidizable and residual fractions for Cu was very high (88.8%). The same result could be attained in the residues. Statistical analysis shows that at low temperatures the variation in pyrolysis temperature did not effectively contribute to the distribution of metal speciation in the residues. Meanwhile a modified Toxicity Characteristic Leaching Procedure (TCLP) was employed to determine the leachability of these four metals. The result indicates that the TCLP concentration of Cu, Zn, Pb and Cd dropped sharply after the temperature reached 350 °C, 550 °C, 500 °C and 400 °C respectively, which means pyrolysis can enhance the stability of these four metals when the temperature is high enough.

Speciation and environmental risk assessment of heavy metal in bio-oil from liquefaction/pyrolysis of sewage sludge.

Liquefaction bio-oil (LBO) produced with ethanol (or acetone) as the solvent and pyrolysis bio-oil (PBO) produced at 550°C (or 850°C) from sewage sludge (SS) were produced, and were characterized and evaluated in terms of their heavy metal (HM) composition. The total concentration, speciation and leaching characteristic of HMs (Cu, Cr, Pb, Zn, Cd, and Ni) in both LBO and PBO were investigated. The total concentration and exchangeable fraction of Zn and Ni in bio-oils were at surprisingly high levels. Quantitative risk assessment of HM in bio-oils was performed by the method of risk assessment code (RAC), potential ecological risk index (PERI) and geo-accumulation index (GAI). Ni in bio-oil produced by pyrolysis at 850°C (PBO850) and Zn in bio-oil by liquefaction at 360°C with ethanol as solvent (LBO-360E) were evaluated to possess very high risk to the environment according to RAC. Additionally, Cd in PBO850 and LBO-360E were evaluated by PERI to have very high risk and high risk, respectively, while Cd in all bio-oils was assessed moderately contaminated according to GAI.

Source identification and potential ecological risk assessment of heavy metals in PM2.5 from Changsha

The probable sources and potential ecological risks of Cu, Zn, Cd and Pb in PM2.5 in Changsha were analyzed. The intelligent medium-flow total suspended particle sampler was used to collect the PM2.5 samples from Yuelu (YL), Kaifu (KF), and Yuhua (YH) districts of Changsha in March-April of 2013. The total concentration of heavy metals (HMs) in PM2.5 was used for source identification by correlation coefficients and principal component analysis (PCA). Otherwise the potential ecological risks indices (RIs) were calculated based on the bioavailable fractions of HMs which were obtained through BCR sequential extraction. Almost 50% of Cu, Cd and Pb in PM2.5 of all sites were accumulated in soluble and reducible fractions by speciation analysis. The correlation coefficients and PCA analysis showed that HMs in PM2.5 of Changsha in spring were mainly from vehicular emissions, fuel combustion, resuspension of dust and other pollution sources. The average potential ecological RIs of HMs in PM2.5 of Changsha were 6193.80 which suggests that HMs in PM2.5 was extremely serious. These results would be a good reference for health studies and formulation of environmental regulations.

Mass concentration and health risk assessment of heavy metals in size-segregated airborne particulate matter in Changsha

This study was performed to investigate the concentration and the health risk of heavy metals (HMs: Zn, Pb, Cd, Ni, Fe, Mn, Cr and Cu) in size-segregated airborne particulate matter (APM). APM samples were collected into 9 size fractions (>9.0 μm, 5.8-9.0 μm, 4.7-5.8 μm, 3.3-4.7 μm, 2.1-3.3 μm, 1.1-2.1 μm, 0.7-1.1 μm, 0.4-0.7 μm, <0.4 μm) by an 8 Stage Non-Viable Cascade Impactor in the campus of Hunan University in Changsha. And then 9 fractions of APM were analyzed for HMs by ICP-OES. The total size-segregated APM concentration in the campus of Hunan University ranged from 120.24 to 271.15 μg/m(3), and the concentration of HMs in APM was in the range of 38.08-13955.14 ng/m(3). The health risk of HMs in APM was evaluated by hazard quotient (HQ) and hazard index (HI) and the results showed that dermal contact and ingestion of APM were the major exposure pathways to human health. The HI values of Cd, Mn, Pb and Cr for children and Cd, Mn and Pb for adults exhibited to be higher than 1 indicating that a non-carcinogenic health effect existed in the APM of the campus of Hunan University. The carcinogenic risks of Cd, Ni and Cr were all bellow the safe value.

Pyrolysis characteristics and kinetics of sewage sludge for different sizes and heating rates

The pyrolysis characteristics and kinetics of sewage sludge for different sizes (dxa0<xa00.25xa0mm, 0.25xa0mmxa0<xa0dxa0<xa00.83xa0mm, and dxa0>xa00.83xa0mm) and heating rates (5, 20, and 35xa0°C/min) were investigated in this article. The STA 409 was utilized for the sewage sludge thermogravimetric analysis. FTIR analysis was employed to study the functional groups and intermediates during the process of pyrolysis. Meanwhile, a new method was developed to calculate pyrolysis kinetic parameters (activated energy E, the frequency factor A, and reaction order n) with surface fitting tool in software MATLAB. The results show that all the TG curves are divided into three stages: evaporation temperature range (180–220xa0°C), main decomposition temperature range (220–650xa0°C), and final decomposition temperature range (650–780xa0°C). The sewage sludge of dxa0<xa00.25xa0mm obtains the largest total mass loss, especially at the heating rate of 5xa0°C/min. By FTIR analysis, the functional groups including NH, C–H, C=C, etc., are all found in the sewage sludge. There is a comparison between the FTIR spectra of sludge heated to 350xa0°C (temperature associated to maximum devolatilization rate in the second stage) and the FTIR spectra of sludge heated to 730xa0°C (temperature associated to maximum devolatilization rate in the third stage). In the second stage, the alcohols, ammonia, and carboxylic acid in the sludge have been mostly decomposed into gases, and only a little bit of compounds containing CH and OH of COOH exist. The pyrolysis kinetic parameters of second stage are as follows: the reaction orders are in the range of 1.6–1.8 and the activation energy is about 45xa0kJ/mol. The frequency factor increases with the increase of heating rate and sewage sludge size.

Fate and risk assessment of heavy metals in residue from co-liquefaction of Camellia oleifera cake and sewage sludge in supercritical ethanol.

The fate and risk assessment of heavy metals (HMs) in solid residue from co-liquefaction of sewage sludge (SS) and Camellia oleifera cake (COC) in supercritical ethanol (SCE) were investigated. SCE effectively stabilized HMs in solid residues and a better stabilization was presented on Zn than Cd. Moreover, SCE significantly transformed Cd, Cu and Zn into F4, which reduced the risk to the environment. Furthermore, risk assessments of Igeo, Er(i), RI and RAC demonstrated that the addition of COC was beneficial to the contamination decrement of HMs since pollution levels of HMs all decreased after treatment, and the lowest pollution level was obtained with SC-350. Therefore, SS treated by SCE with the addition of COC could be a promising technology for disposal of SS, especially considering the safety of COC as regards HMs problem.

Selective catalytic reduction (SCR) of NO by urea loaded on activated carbon fibre (ACF) and CeO2/ACF at 30 °C: The SCR mechanism

Selective catalytic reduction (SCR) of NO by urea loaded on rayon-based activated carbon fibre (ACF) and CeO2/ACF (CA) was studied at ambient temperature (30 °C) to establish a basic scheme for its reduction. Nitric oxide was found to be reduced to N2 with urea deposited on the ACF and CA. When oxygen was present, the greater the amount of loaded urea (20–60%), the greater the NOx conversions, which were between 72.03% and 77.30%, whereas the NOx conversions were about 50% when oxygen was absent. Moreover, when the urea was loaded on CA, a catalyst containing 40% urea/ACF loaded with 10% CeO2 (UCA4) could yield a NOx conversion of about 80% for 24.5 h. Based on the experimental results, the catalytic mechanisms of SCR with and without oxygen are discussed. The enhancing effect of oxygen resulted from the oxidation of NO to NO2, and urea was the main reducing agent in the SCR of loaded catalysts. ACF-C was the catalytic centre in the SCR of NO of ACF, while CeO2 of urea-loaded CA was the catalytic centre.

Room temperature removal of NO by activated carbon fibres loaded with urea and La2O3

In this paper, catalytic samples of 10, 20, 30, 40 and 50% (w/w) urea/activated carbon fibre (AFC), 10% urea–5% La2O3/ACF, 10% urea–10% La2O3/ACF, 10% urea–15% La2O3/ACF, 20% urea–5% La2O3/ACF, 20% urea–10% La2O3/ACF, and 20% urea–15% La2O3/ACF were prepared and used for removal of NO under the condition of: NO, 500 ppm; O2, 21%; N2, balance, gas space velocity=10000 m3·h−1·m−3, total gas flow = 266.7 mL min−1, temperature = 30°C, relative humidity=0%. The physical and chemical properties of the prepared catalysts were characterized by surface area measurements (BET) and scanning electron microscopy studies. Furthermore, the catalytic stability of 10% urea–5% La2O3/ACF under different concentrations of NO and O2 were also studied. The results showed that, among the prepared urea/ACF samples, 20% urea/ACF yielded the highest NO conversion at room temperature. Meanwhile, among the prepared urea–La2O3/ACF catalysts, 10% urea–5% La2O3/ACF yielded the highest NO conversion. Both 20% urea/ACF and 10% urea–5% La2O3/ACF could yield over 95% NO conversion at ambient temperature. However, 10% urea–5% La2O3/ACF had a more stable activity than that of 20% urea/ACF. The catalytic and characterization experimental results, including BET, thermogravimetric analysis and Fourier transform infrared analysis, showed that the NO selective catalytic reduction mechanism of urea–La2O3/ACF was different from that of ACF and urea/ACF. The NO was purified by ACF mainly by adsorption, whereas there was mainly a reduction reaction when NO was purified by urea/ACF or urea–La2O3/ACF. ACF-C was not only the catalyst but also the reducing agent for urea/ACF, whereas, for urea–La2O3/ACF, the catalytic centre was La2O3, and ACF was mainly the carrier. These differences resulted in the higher and more stable NO removal by 10% urea–5% La2O3/ACF.

Digested sewage sludge gasification in supercritical water

Digested sewage sludge gasification in supercritical water was studied. Influences of main reaction parameters, including temperature (623–698 K), pressure (25–35 Mpa), residence time (10–15 min) and dry matter content (5–25 wt%), were investigated to optimize the gasification process. The main gas products were methane, carbon monoxide, carbon dioxide and traces of ethene, etc. Results showed that 10 wt% dry matter content digested sewage sludge at a temperature of 698 K and residence time of 50 min, with a pressure of 25 MPa, were the most favorable conditions for the sewage sludge gasification and carbon gasification efficiencies. In addition, potassium carbonate (K2CO3) was also employed as the catalyst to make a comparison between gasification with and without catalyst. When 2.6 g K2CO3 was added, a gasification efficiency of 25.26% and a carbon gasification efficiency of 20.02% were achieved, which were almost four times as much as the efficiencies without catalyst. K2CO3 has been proved to be effective in sewage sludge gasification.