Bibo Xu's research while affiliated with U.S. Department of Housing and Urban Development and other places

This work had demonstrated a novel and high-efficiency perchlorate reduction particle electrode using benzalkonium chloride as dynamics strengthener and coupling with rhenium (Re) and palladium (Pd) nanoparticles (Re-Pd/BC). Perchlorate ions could be rapidly adsorbed in the presence of benzalkonium chloride, facilitating its subsequent reduction on the Re-Pd/BC particle electrode. On the basis of the characterization results and kinetics analysis, a synergistic effect of Re and Pd was observed, perchlorate could be efficiently reduced and completely converted into chlorine in optimized conditions (pH 3.0, anaerobic and current density 20 mA/cm²) and the reduction rate constant of Re-Pd/BC was 0.9451 L⁻¹ gcat⁻¹. The superfluous oxygen in solution could lead to Re-Pd/BC deactivation, and increased current density was beneficial of electro-reduction. The increased pH lead to decrease of reduction efficiency, and the coexisting anion of nitrate could be competed with perchlorate for reduction sites which lead to decrease of reduction rate. Based on the experimental results, it was found that the existing form of Re and Pd on the surface of Re-Pd/BC, the number of atomic H∗ and current density played an important role in perchlorate electro-reduction process.

In this study, hydrothermal carbonization (HTC) at 200 °C and 250 °C was combined with acetic acid and NaOH (changing feedwater pH from 2-12) to treat woody biomass for enhancing its pelletization and fuel properties. The treated biomass (hydrochar) was characterized by composition analysis and FTIR, after which the energy consumption of pelletization; tensile strength; fuel properties, including proximate, elemental analysis, and mass density; and combustion characteristics of corresponding pellets were analyzed. The results showed that both the acetic acid and NaOH-aided HTC processes decreased the energy consumption of hydrochar compression. Weakly basic or acidic feedwater decreased the tensile strength of hydrochar pellets, but it increased at pH 2 and 12. The changes in treatment pH from 4 to 9 had little effect on the O/C and H/C content of the hydrochar pellets, while the addition of acetic acid enhanced oxygen and hydrogen loss more than NaOH. The heating values of hydrochar pellets produced at pH 2 from 200 °C and 250 °C were 20.7 and 22.8 MJ/kg, respectively. Hydrochar pellets produced from HTC at 250 °C and pH 2 had higher mass density and lower equilibrium water content than those produced under other conditions. The combustion process was promoted with reduced ignition temperature, maximum weight loss rate, and burnout temperature. These findings demonstrate that acetic acid-aided HTC of woody biomass was more suitable than NaOH for producing hydrochar pellets for use as a biofuel.

The influence of temperature (180-260°C) on the fate of nitrogen during hydrothermal carbonization (HTC) of food waste (FW) was assessed. The distribution and evolution of nitrogen in aqueous products and bio-oil, as well as hydrochar, were conducted. Results suggested that elevated temperature enhanced the deamination and the highest ammonium concentration (929.75mg/L) was acquired at 260°C. At temperatures above 220°C, the total N in the hydrochar became stable, whereas the mass percentage of N increased. Amines and heterocyclic-N compounds from protein cracking and Maillard reactions were identified as the main nitrogen-containing compounds in the bio-oil. As to the hydrochar, increasing temperature resulted in condensed nitrogen-containing aromatic heterocycles (e.g. pyridine-N and quaternary-N). In particular, remarkable Maillard reactions at 180°C and the highest temperature at 260°C enhanced nitrogen incorporation (i.e. quaternary-N) into hydrochar.

In this study, the effect of feedwater pH (3-11) on phosphorus (P) transformation during the hydrothermal carbonization (HTC) of sewage sludge (SS) was investigated at a temperature range of 200-260°C. The HTC significantly accumulated P in the hydrochar. Different feedwater pH stimulated the transformation of various forms of P. An acidic feedwater pH promoted the transformation of apatite phosphorus (AP) to non-apatite inorganic phosphorus (NAIP), and of organic P (OP) to inorganic P (IP). The NAIP tended to transformation to AP and a small part of the IP was transformed to OP when the SS was treated in a basic environment. The combination of three P analysis methods (chemical extractive fractionation, X-ray powder diffraction (XRD) and energy dispersive spectroscopy (EDS)) showed that metal cations (e.g. Al and Ca) and the pH played important roles in the transformation of different forms of P during the HTC of the SS.

To investigate the structure and reaction pathway of char, experiments were conducted in a reaction vessel with nitrogen gas using hydrothermal treatment (HTT) to produce char from sewage sludge (SS) with sawdust (SD), corncobs (CC) and cornstalks (CS) as raw feed stock. The HTT was conducted at temperatures ranging from 220 °C to 300 °C. Elemental analysis, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectra were used to identify the composition, structure, and functional groups of the char. The results show that the H/C and O/C atomic ratios of char decreased as the reaction temperature increased, and the lowest values, 0.91 and 0.04, respectively, were obtained at 300 °C. After HTT, C–(C,H) hydrocarbon in carbohydrates, proteins and lipids gradually depolymerised to C–H. In particular, for the char derived from SS with SD, several long aliphatic chains were obtained. Moreover, dramatic hydrolysis of amide, as well as decarboxylation, occurred at 260 °C. C[dbnd]N bonds were gradually broken with amide hydrolysis. The aromatisation reaction occurred as the –C[dbnd]C group was enhanced slightly after HTT. In general, the carbon groups of char condensed from the small aromatic ring system to large aromatic ring systems.

Hydrothermal carbonisation (HTC) of sewage sludge (SS) with waste biomass was investigated as a clean and energy-efficient treatment to produce char. The effects of the reaction temperature on the properties, composition, and energy consumption of the obtained char were investigated to evaluate the feasibility of the production process. The results indicate that the dewaterability of char derived from SS with biomass was enhanced by approximately 50% at temperatures exceeding 260°C. The lowest moisture content of the char was 41.39%, produced from SS with cornstalk at 300 °C and holding time of 60 min. The values of H/C and O/C in char from SS with sawdust, corncob and rape straw at 300 °C dropped to approximately 0.92 and 0.04, respectively, which are close to the values of bituminous coal. SS mixed with corncob was more efficient than other biomass waste during the HTC process. The suggested optimum condition to produce char is 300 °C for 60 min, in which the HHV and energy recovery rate can reach 21.31 MJ/kg and 71.60%, respectively. As regards other types of biomass, a moderate reaction temperature above 260°C is suggested to produce chars with an energy recovery rate ranging from 47.06% to 71.60%.

Biomass activated carbons were activated and pyrolyzed at 300-800 °C under nitrogen circumstance (named as CAC300-CAC800). The physicochemical characteristics of CACs and mechanisms of perchlorate (ClO4⁻) adsorption on CACs were investigated. Compared to CAC300, CAC600-CAC800 had higher surface area, total prove volume, higher hydrophobic surface and an abundant of oxygen-containing groups (-COH and -COOH) which contributed to a higher ClO4⁻ adsorption capacity. The pH value of solution significantly affected ClO4⁻ adsorption and the maximum ClO4⁻ adsorption capacity occurred at pH 2.0 (less than pHIEP), where surface charge was positive. Electrostatic attraction and hydrogen bonding to oxygen-containing groups on CACs were considered as the dominant force for ClO4⁻ adsorption. With the pH increased, electrostatic attraction disappeared and surface oxygen-containing groups were gradually deprotonated, and the ClO4⁻ adsorption amount was gradually decreased few until pH 12.0. The batch adsorption experiments of ClO4⁻ showed that CAC600-CAC800 were effective adsorbents for the adsorption of ClO4⁻.

This study was performed to investigate pollution of traffic-related heavy metals (HMs-Zn, Pb, Cu, Cr, and Cd) in roadside soils and their uptake by wild plants growing along highways in Hunan Province, China. For this, we analyzed the concentration and chemical fractionation of HMs in soils and plants. Soil samples were collected with different depths in the profile and different distances from highway edge. And leaves and barks of six high-frequency plants were collected. Results of the modified European Community Bureau of Reference (BCR) showed that the mobile fraction of these HMs was in the order of Cd > Pb > Zn > Cu > Cr. A high percentage of the mobile fraction indicates Cd, Pb, and Zn were labile and available for uptake by wild plants. The total concentration and values of risk assessment code (RAC) showed that Cd was the main risk factor, which were in the range high to very high risk. The accumulation ability of HMs in plants was evaluated by the biological accumulation factor (BAF) and the metal accumulation index (MAI), and the results showed that all those plant species have good phyto-extraction ability, while accumulation capacity for most HMs plants tissues was bark > leaf. The highest MAI value (5.99) in Cinnamomum camphora (L) Presl indicates the potential for bio-monitoring and a good choice for planting along highways where there is contamination with HMs.

This study investigated how two factors, extraction solvent and time, affected extraction efficiencies of ultrasonic extraction (UE) for polycyclic aromatic hydrocarbons (PAHs). The initial extraction has great impact on the final extraction efficiencies of PAHs, and further affects the quantitative analysis of PAHs. In this study, the former factor inspected parameters of type of extraction solvent, composition of solvent, and solvent dosage, while the latter concerned UE duration per cycle and number of UE cycles. Results showed that single solvents, n-hexane, dichloromethane (DCM), and acetone, provided average recovery rates of 53.76% ± 63.21% (average value ± standard deviation), 37.81% ± 19.48%, and 67.66% ± 60.76%, respectively, for the total PAHs targeted. However, binary solvents, n-hexane/DCM (1:1), DCM/acetone (1:1), and acetone/n-hexane (1:5), yielded average recovery rates of 112.08% ± 39.23%, 82.56% ± 48.78%, and 52.77% ± 30.45%, respectively, for the total target PAHs. The three single solvents yielded average recovery rates of 0–22.32% for 3-ring PAHs, 6.01–41.76% for 4-ring PAHs, and 46.53–85.71% for 5- and 6-ring PAHs, while the three binary solvents provided 29.64–31.00%, 19.65–40.62%, and 28.38–49.78% for the three types of PAHs, respectively. The other three parameters supplied with similar recovery rates for these types of PAHs. When applying n-hexane/DCM (1:1) as PAH extraction from real airborne particulate matter, the three types of PAHs occupied 9.31%, 21.40%, and 69.29% over total PAHs, respectively.