In present study, the internal flow field of expanded granular sludge bed (EGSB) reactors was investigated using CFD simulation. A two-dimensional Eulerian three phases (wastewater, biogas and sludge) fluid model was applied to investigate the impact of baffle angle (θ) on the separation efficiency and the hydraulic characteristics in three-phase separation zone. The results showed that when θ was 40°, the sludge loss rate was the smallest and the reflux condition of sludge was the best. To verify the practical applicability of the fluid model, a tracer experiment was conducted. A good relationship between simulation and experiment was discovered, which further confirmed that the baffle angle has significant impact on the separation efficiency of the three-phase separator.

•Large amounts of Na, Al and Si dissolved from alkali-thermal-activated RM in water.•Low Si/Al geopolymers will dissolve and release soluble Al in long-term curing.•Dissolved Si from SF participates in the polymerization of soluble Na, Al and Si.•The binding energies of AlO and SiO will increase after geopolymerization.•The coordinated Fe3 + replaces Al3 + in aluminosilicate structure of geopolymer.Reaction mechanism of one-part geopolymer formation using red mud (RM) was explored in this study. Leaching test, X-ray photoelectron spectroscopy (XPS), Mössbauer spectroscopy and nitrogen isothermal adsorption (NIA) were conducted to study transformations of Na, Al, Si, Fe species, binding energies of AlO and SiO, and pore structure, respectively. The alkali-thermal-activated RM could dissolve in water to release large amounts of soluble Na, Al and Si species for geopolymerization. But more dissolved Si is needed from added silica fume (SF) to form stable geopolymer gels. The binding energies of AlO and SiO bonds in the geopolymer increased with the polymerization of reactive SiO2 and Al2O3. The coordinated Fe3 + in RM played a role like Al3 + by replacing it in the aluminosilicates structure, and exerted no obvious impact on geopolymerization. The dissolved SF took part in the geopolymerization to form dense geopolymer matrices in long-term curing.Download high-res image (145KB)Download full-size image

•The performance of the EGSB reactor was modeled by an artificial neural network.•The topologic architecture of the artificial neural network was optimized by response surface methodology.In present study, a three-layer backpropagation neural network (BPNN) model was developed to predict the performance of an expanded granular sludge bed (EGSB) reactor. Six related variables such as influent chemical oxygen demand (COD) concentration, hydraulic retention time (HRT), alkalinity (ALK) concentration, pH, volatile fatty acid (VFA) concentration and oxidation reduction potential (ORP), were selected as inputs of the model. All input values were converted to the range (−1, 1) before passing them into the network. Activation function of hidden layer and output layer were “tansig” and “purelin” individually. Several comparisons were conducted to obtain an optimal network structure. Dividerand function was chosen to divide the operating data into training group, testing group and validation group. The Levenberg Marquardt algorithm (trainlm) was found as the best of the ten training algorithms. Other model parameters such as number of neurons in the hidden layer (X1), initial adaptive value (X2) and initial value of weights and biases (X3) were optimized using response surface methodology (RSM). The optimum conditions for minimum mean squared error (MSE) were as follows: X1 (12), X2 (6.0) and X3 (1.0). The precision of optimum ANN model was assessed by means of various statistics such as MSE, determination coefficient (R2), coefficient of variation (CV) and MSE. The result indicated that the proposed ANN model exhibited superior predictive accuracy for the forecast of COD removal performance by EGSB system. Finally, the results of connection weights method demonstrated that VFA concentration (50.37%) had a remarkable impact on reactor performance.

•A S curve model has been proposed in the EDW process.•The S curve model comprise desalination, dewatering, and dewaterability limit.•The optimal dosage of Na2SO4 for improved EDW was 12.5 g kg−1 DS.•The migration of water, Na+, and organic matters were studied in three cake layers.•The negative effects of electromigration and electrolysis on EDW were discussed.In this study, the influence of Na2SO4 on electro-dewatering (EDW) of waste activated sludge (WAS) was investigated. The highest water removal efficiency of 42.5% was achieved at the optimum Na2SO4 dosage of 12.5 g kg−1 DS during EDW process at a constant voltage of 20 V. The migration and distribution of water, organic matters and Na+ at different Na2SO4 dosages were investigated through layered experiments. The results indicated the entire EDW process followed the S curve model, and it can be divided into three stages: (1) initial desalination stage: at the initial few min of EDW process, the rate of electroosmosis was extremely slow while electromigration of ions like Na+ was intense, and the electromigration was more obvious with increased Na2SO4 dosage; (2) dewatering stage: the dewatering efficiency increased dramatically via electroosmosis; (3) the dewaterability limit stage: the maximum value of dewatering efficiency has been achieved, while the water removal efficiency and dry solids content remained constant. During the EDW process, the possible electrolysis resulted in a pH gradient in the sludge cake. With the addition of Na2SO4 in the EDW, the pH gradient was intensified, and the migration rate of organic matters moving from cathode to anode increased while compared with the raw WAS. This study provided insights into the mechanism of EDW process at different dosages of Na2SO4.Download high-res image (309KB)Download full-size image

•High-voltage electrochemical workstation instead of traditional DC power source.•Voltage contribution analysis used by in-situ linear sweep voltammetry (LSV).•EDW mode could be explained by the Ohm's law.•Mass of filtrate is linear proportional to the amount of electric charge.Electro-dewatering (EDW) is an innovative method for volume reduction of sewage sludge before re-utilization and disposal. In this study, dewaterability limit and energy consumption in sludge electro-dewatering process were directly explored using in-situ linear sweep voltammetry (LSV) analysis by a high-voltage electrochemical workstation instead of a traditional DC power source. Dewaterability limits of biosolids EDW were identified under a constant-voltage dewatering mode with different applied voltages at 10, 20, 30, 40, and 50 V, independently. The LSV tests reveal that the dewaterability limit of sludge is attributed to the higher electrical resistance of the sludge layer near the anode. The mass of the filtrate flow was linearly proportional to the total amount of electric charge corresponding to the energy consumption in EDW, which elucidated the principal mechanism of EDW. Under a constant-voltage dewatering mode, the applied voltage is a key factor in controlling the energy consumption. Reducing the applied voltage while extending the dewatering time is proposed to reduce the energy consumption and obtain a good dewatering result. As the applied voltage decreased from 50 to 10 V, the energy consumption could be reduced from 403.6 to 80.3 kWh/m3 removed water, the dewatering times was increased from 6.4 to 85.4 min, and the dry solids content of dewatered cake increased from 16.1 to 34.4 wt%. Those results indicate that LSV is an effective method to reveal the mechanism of EDW and optimize the operation parameters to reduce energy consumption.Download high-res image (266KB)Download full-size image

•Leady oxides synthesized from lead citrate precursor in different atmosphere.•Porous carbon in leady oxides was firstly investigated.•Mass percentage of Pb metal and carbon in leady oxides can be controlled.•A calcination theory model of mass-transfer controlled is proposed.A novel nanostructured leady oxides comprising porous carbon skeleton has been synthesized by thermal decomposition of lead citrate precursor, recovered from spent lead-acid battery paste. The influences of O2 percentage in the calcination atmosphere (O2/N2 mixture) and the temperature on leady oxide product characteristics are studied by chemical analysis, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The major crystalline phases of the products are identified as lead oxides, metallic Pb, and carbon. Porous carbon is observed as skeletons within the leady oxide (PbO containing some Pb metal) particles. Mass percentage of Pb metal in the leady oxide increases with increasing the proportion of oxygen in the calcination atmosphere. However, the amount of carbon decreases from approximately 8.0 to 0.3 wt%, and the porous carbon skeleton structure is gradually damaged with oxygen concentration increasing. A model about the thermal decomposition of lead citrate precursor is firstly proposed to elucidate these observations. The nanostructured leady oxides combined with porous carbon can be directly used as precursor of active materials in a new lead acid battery.

In this study, a granular adsorbent was prepared from phoenix tree leaf powder with bentonite as the binder. The granular adsorbent was characterized by TG, BET and SEM analyses. The maximum specific surface area and pore volume were 166.3 m2 g−1 and 0.276 cm3 g−1, respectively, after the granular adsorbent was calcined at 500 °C. Effects of pH, adsorption time and initial metal ion concentration on the adsorption of Pb2+ by 500 °C calcined granular adsorbent were investigated in batch experiments. Higher pH was favorable for the adsorption process and significant release of Na+, K+ and Mg2+ were observed, assuming the predominant Pb2+ adsorption mechanism was ion exchange. The adsorption could attain equilibrium within 24 h with a gradual increase of the solution pH. The kinetics data were analyzed using three adsorption kinetic models: the pseudo-first-order, pseudo-second-order and intraparticle diffusion equations. Results show that intraparticle diffusion or chemical adsorption is the rate-limiting step depending on the adsorption time. The adsorption isotherms best fitted the Langmuir–Freundlich model and the maximum Langmuir adsorption capacity was found to be 71 mg g−1. This novel granular adsorbent has proven to be a potential inexpensive adsorbent for Pb2+ removal from aqueous solutions.

•Pyrolysis of sludge cake conditioned with Fenton's reagent and red mud was firstly studied.•Red mud can enhance gas production, especially H2 yield.•The higher the iron oxide content in the red mud, the higher the yield of H2 in pyrolysis.•Red mud enhanced the char conversion and tar re-formation which led to higher H2 yield.This study investigated the pyrolytic performances of deep-dewatered sludge cakes which were previously conditioned with Fenton's reagent and red mud. The pyrolytic products, including fuel gas, tar and solid char, were characterized by GC, GC-MS and FTIR. The results showed that pyrolysis of sludge cakes conditioned with Fenton's reagent and red mud produced more gas, especially higher H2 yield, compared with pyrolysis of raw sludge. In addition, red mud promoted PAHs formation and tar re-formation through side-chain-cleaving, dehydrogenation, polycondensation and aromatization of hydrocarbon during the pyrolysis, resulting in enhancement of the H2 yield. The sharp decrease in the absorbance of CHaromatic group of chars from the conditioned cakes implied that red mud could intensify the cracking and re-formation of aromatics. The higher the iron oxide content in red mud was, the better catalytic efficiency in the decomposition of organics and tar re-formation was, which led to more H2 production.

•One-part geopolymer was synthesized by using Bayer red mud as main raw material.•Long-term strength of binder was significantly improved with addition of 20–30 wt% SF.•Lower water/solid ratio contributed to increasing the strength.•The compressive strength of geopolymer cured for 28 d reached 31.5 MPa.•Geopolymerization of dissolved aluminosilicate and silica formed dense matrices.One-part geopolymer was synthesized from alkali–thermal activated Bayer red mud (RM) with addition of silica to optimize its composition. The RM was pretreated through alkali–thermal activation and turned to geopolymer precursor, which could be used by only adding water in blending process. However the long-term strength of the binder with only RM was poor because of the unstable polymerization due to the low SiO2/Al2O3 molar ratio (1.41). Silica fume (SF) was chosen to increase the SiO2/Al2O3 molar ratio of the geopolymer formulation. By adding 25 wt% of SF, the 28 d compressive strength of the geopolymer with a SiO2/Al2O3 molar ratio of 3.45 could reach 31.5 MPa at a water/solid ratio of 0.45. Sodium aluminosilicate in the activated RM dissolved in water and formed an alkaline environment to dissolve SF. The dissolved silica participated in geopolymerization, leading to a satisfactory geopolymer composition. Typical amorphous geopolymer matrices were formed in the binder completely cured.

•The optimal addition of SnSO4 in electrolyte of lead acid battery is 2.24 mmol L−1.•The novel leady oxides are used as active material of working electrode.•The novel leady oxides are used as active material of positive plate.•SnSO4 as an electrolyte additive could effectively decrease crystal particles in active materials.•SnSO4 as an electrolyte additive has a positive effect on restriction of irreversible sulfation.The effects of SnSO4 as an electrolyte additive on the microstructure of positive plate and electrochemical performance of lead acid battery made from a novel leady oxide are investigated. The novel leady oxide is synthesized through leaching of spent lead paste in citric acid solution. The novel leady oxides are used to prepare working electrode (WE) subjected to electrochemical cyclic voltammetry (CV) tests. Moreover, the novel leady oxides are used as active materials of positive plate assembled as a testing battery of 1.85 A h capacity. In CV tests, SEM/EDX results show that the major crystalline phase of the paste in WE after CV cycles is PbSO4. The larger column-shaped PbSO4 crystals easily generate in the paste of WE without an electrolyte additive of SnSO4. However, PbSO4 crystals significantly become smaller with the addition of SnSO4 in the electrolyte. In batteries testing, SEM results show that an electrolyte additive of SnSO4 could effectively decrease PbO2 particle size in the positive active materials of the teardown battery at the end of charging procedure. It is indicated that an electrolyte additive of SnSO4 could have a positive influence on restraining larger particles of irreversible sulfation in charge/discharge cycles of battery testing.

Pilot-scale sewage sludge dewatering experiments were conducted using two composite conditioners: FeCl3 + lime (Fe-lime) and Fenton's reagents + red mud (Fenton-RM). Mass balance analysis was performed on Cu, Zn, Pb, Cd, and Cr during the conditioning and dewatering processes to investigate their transformation and distribution. Speciation of heavy metals was also investigated by the Tessier sequential extraction method. Results show that (1) most of the heavy metals were retained in the solid cake during the dewatering process, especially Cu and Cr; (2) in the sludge cakes, more than 87 wt% of Cu and Cr existed in organic bound and residual forms, and the contents of the bioavailable fractions (exchangeable, carbonate bound and Fe–Mn oxides bound form) for Zn, Pb, and Cd were larger in the Fenton-RM system than those in the Fe-lime system; (3) the main factors affecting the distributions of these heavy metals are pH and chemical speciation in two conditioning processes. Generally, heavy metal concentrations in filtrate and dewatered sludge cakes in both systems were below the corresponding standards, and heavy metals in the dewatered sludge cake could be more effectively immobilized in the Fe-lime system than in that of the Fenton-RM system.

•Novel leady oxide is prepared from Pb3(C6H5O7)2·3H2O, recovered from spent lead pastes.•Properties of leady oxides, which affect battery performance, change with calcination temperature.•Leady oxide prepared at 375 °C exhibits excellent initial capacity as PAM.A novel ultrafine leady oxide has been prepared from a combustion–calcination process of lead citrate precursor (Pb3(C6H5O7)2·3H2O), by hydrometallurgical leaching of spent lead pastes firstly. The leady oxides are used to assemble lead acid battery which are subjected to cyclic voltammetry (CV) and battery testing. Various key properties of the new oxides, such as morphology, crystalline phases, degree of oxidation, apparent density and water and acid absorption value have been characterized by chemical analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that leady oxides synthesized at different calcination temperatures mainly comprise β-PbO, α-PbO and Pb. Unlike traditional leady oxide, the new oxide product prepared at 375 °C has a rod-like morphology with greater porous structure, and appears smaller density, lower value of acid absorption and larger propensity for water absorption. In battery testing, the 20 h rate and 1C rate discharge time have exceeded 26 h and 40 min, respectively. Results reveal that the leady oxide prepared at 375 °C exhibits excellent electrochemical performance and initial capacity as positive active material. While leady oxide obtained at 450 °C presents a relatively improved cycle life. Further work is to optimize the battery manufacturing process for better cycle performance.

•The Pb(CH3COO)2·3H2O precursor was prepared from the spent lead battery pastes.•Novel lead oxide products were prepared from the precursor in N2 and air.•The assembled batteries show a good cyclic stability in 80 charge/discharge cycles.A novel green recycling process is investigated to prepare lead acetate trihydrate precursors and novel ultrafine lead oxide from spent lead acid battery pastes. The route contains the following four processes. (1) The spent lead pastes are desulphurized by (NH4)2CO3. (2) The desulphurized pastes are converted into lead acetate solution by leaching with acetic acid solution and H2O2; (3) The Pb(CH3COO)2·3H2O precursor is crystallized and purified from the lead acetate solution with the addition of glacial acetic acid; (4) The novel ultrafine lead oxide is prepared by the calcination of lead acetate trihydrate precursor in N2 or air at 320–400 °C. Both the lead acetate trihydrate and lead oxide products are characterized by TG-DTA, XRD, and SEM techniques. The calcination products are mainly α-PbO, β-PbO, and a small amount of metallic Pb. The particle size of the calcination products in air is significantly larger than that in N2. Cyclic voltammetry measurements of the novel ultrafine lead oxide products show good reversibility and cycle stability. The assembled batteries using the lead oxide products as cathode active materials show a good cyclic stability in 80 charge/discharge cycles with the depth of discharge (DOD) of 100%.

•Calcium sulfate whiskers (CSWs) were prepared from purified FGD gypsum in H2SO4–NaCl–H2O system.•Crystallization of CSW could be controlled via the formulas of H2SO4 and NaCl.•H2SO4 and NaCl have opposite effects on the solubility of FGD gypsum.Little attention has thus far been paid to the potential effect of solution composition on the hydrothermal crystallization of calcium sulfate whiskers prepared from flue-gas desulfurization (FGD) gypsum. When purified FGD gypsum was used as raw material, the morphology and phase structure of the hydrothermal products grown in pure water, H2SO4–H2O, NaCl–H2O, and H2SO4–NaCl–H2O solutions as well as the solubility of purified FGD gypsum in these solutions were investigated. The results indicate that calcium sulfate whiskers grow favorably in the H2SO4–NaCl–H2O system. When prepared using 10–70 g NaCl/kg gypsum −0.01 M H2SO4–H2O at 130 °C for 60 min, the obtained calcium sulfate whiskers had diameters ranging from 3 to 5 μm and lengths from 200 to 600 μm, and their phase structure was calcium sulfate hemihydrate (HH). Opposing effects of sulfuric acid and sodium chloride on the solubility of the purified FGD gypsum were observed. With the co-presence of sulfuric acid and sodium chloride in the reaction solution, the concentrations of Ca2+ and SO42− can be kept relatively stable, which implies that the crystallization of the hydrothermal products can be controlled by changing the concentrations of sulfuric acid and sodium chloride.

Three methods were used to evaluate hydration reactivity of calcined magnesite products. Both conventional citric acid coloration reactivity test (index of CT) and MgO hydration conversion method (index of X) cannot properly evaluate the reactivity when the magnesite ore decomposes incompletely at lower calcining temperatures. Therefore, a corrected MgO hydration conversion method containing the index of Xcorrected is proposed by considering the decomposition ratio of magnesite ore. Results show that decomposition ratio of magnesite ore is quite low at temperatures lower than 650 °C, which influences the reliability of the conventional reactivity test results (index of CT or X). Reactivity of MgO decreased as the calcining temperature and time increased when the index of Xcorrected was used. Hydration reactivity of calcined magnesite at lower temperatures is much higher than that of magnesite calcined at 1100 °C, no matter which of three evaluation indexes is used (i.e., CT, X, and Xcorrected).

•New construction materials are made from dewatered sewage sludge with autoclave process.•Autoclaved specimens exhibit good long-term performance.•The gel-like and honeycomb-like hydrated products of autoclaved samples are katoite and C–S–H phases.In the present work, we demonstrate an alternative for the final disposal of sewage sludge by using it as an additive in a mixture with cement, fly ash and furnace slag, which can potentially be used to develop newly promising construction materials by autoclave curing. The dewatered sewage sludge is obtained with fly ash and lime. These physical conditioners contribute to both dewatering process and solidifying/stabilizing of sludge.Various mechanical properties such as flexural strength, compressive strength and the toxicity characteristic leaching procedure (TCLP) were evaluated. To evaluate long-term performance, different types of accelerated attacks, i.e. freezing–thawing cycles, accelerated carbonation, wet–dry cycles, and heat–cool cycles were also determined. The obtained test results were indicated that the autoclaved samples exhibit good long-term performance after evaluations of different durability tests. XRD patterns show that the hydration products of autoclaved samples are katoite and C–S–H phases, which mainly contribute to strength of autoclaved products. Morphologies of autoclaved samples also demonstrate the existence of the gel-like and honeycomb-like hydrated products. The results show that this new construction material could be applied as many construction and building materials, i.e. landfill liners and building blocks.

In order to investigate effect of iron on the performance of lead acid batteries, we systematically study the chemical characteristics, electrochemical characteristics, battery capacity and cycle life using iron-doped lead oxide in this article. Cyclic voltammetry results show that positive discharge current decreases sharply with the increasing content of Fe2O3 from 0.05 wt.% to 2 wt.%. The release of H2 and O2 are promoted accompanying the increase of Fe2O3 contents. The chemical analysis confirms that the strength of Fe3+, Fe2+ concentration is simultaneously increased with the increase of iron contents after 50 voltammetry cycles. X-ray diffraction phase analysis shows that the amount of PbSO4 increases with the increasing iron content in the positive plates after 50 discharge cycles. Morphologies of positive plates show that many agglomerates from PbSO4 crystals appear. The SEM observations illustrate that there is a lower porosity and specific surface area in the positive active material with iron after 50 discharge cycles. The mechanism of iron decreasing capacity, cycle-life and promoting the release of H2 and O2 has been elucidated in details. We support it is the “redox-diffusion” process of multiple-valence iron and formation of PbSO4 on electrodes that result in above performances.Highlights• We investigate the effect and mechanism of iron doped lead oxide on batteries. • Iron doped lead oxide can sharply decrease the battery capacity and cycle life. • It can also promote the release of hydrogen and oxygen. • We conclude that “redox-diffusion” process of iron results in bad performances. • We conclude that formation of PbSO4 on electrodes also results in bad performances.

Lead sulfate (PbSO4) is a major component of lead paste of spent lead acid batteries, normally over 60%. In traditional pyrometallurgical process, the decomposition of PbSO4 requires a relatively high temperature and the use of coal as both the reducing agent and as the source of thermal energy, thereby causing emissions of SOx, lead particles and CO2. In this study, lead sulfate was desulfated by adding an aqueous solution comprising citric acid and tri-sodium citrate with the modulation of ethylene glycol (EG) in order to control the morphology. The treating agent reacted with lead sulfate to form an aqueous solution of sodium sulfate and a precipitated precursor of lead carboxylate crystals. The precursor was characterized by XRD, SEM, SDT and FTIR. The morphology of the EG-mediated precursor showed a regular columnar shape. On calcining at relatively low temperatures, EG-mediated precursor was transformed into fine particles of lead oxide. The characteristics of calcined products were investigated by XRD, SEM, and TEM. The results showed that nanostructured crystals of lead oxide could be easily prepared by combustion of EG-mediated precursor at 350 °C with a lead recovery of about 98%. This paper has paved the way for obtaining nanostructured PbO from waste battery scrap.Highlights► Nanostructured lead oxides were prepared by using PbSO4 as a starting material. ► The EG precursor showed the columnar-like morphology with the length of about 10 μm. ► The EG precursor was completely different from the morphology of the CA precursor. ► The calcined products in the size of 50–100 nm were prepared from the EG precursor.