Initial pH Adjustment Research Articles

Overproduction of bacterial cellulose from Acetobacter xylinum BPR2001 using food industries wastes.

In this study, a cost-effective complex culture media containing molasses and corn steep liquor (CSL) was developed for the high production of bacterial cellulose (BC) by investigating the effect of four effective factors on BC production at three levels using Taguchi and combined methods. The predicted and actual values of BC production in optimal conditions by Taguchi and combined methods were 8.41 and 14.52g/L, respectively. These results showed that the combined method was more suitable for predicting the optimal conditions in the optimization of BC production, the cost of developed culture medium was around 94% cost of HS medium preparation, molasses was the most effective factor in both experimental design methods, and initial pH adjustment had little impact on BC production. Then, the effect of inoculation conditions containing three factors of inoculation age, ethanol addition time, and agitation rate on the increase of BC production at three levels was investigated using the response surface methodology with the Box-Behnken design algorithm. Under the optimal conditions including inoculum age of 3 days, ethanol addition time of 10 days, and stirring speed of 100rpm, the predicted and experimental results of BC production were 21.61 and 20.21g/L, respectively. This is among the highest ever reported for BC production, which was achieved with a more cost-effective culture medium containing molasses and CSL.

Lithium recovery from the spent lithium-ion batteries by commercial acid-resistant nanofiltration membranes: A comparative study

Given the critical requirements of environmental preservation and resource reutilization, the recovery of lithium from spent lithium-ion (LIBs) batteries holds immense significance. This study investigates the viability of nanofiltration (NF) membranes for selectively separating lithium from spent LIBs leaching solution. A membrane-based approach uniquely sidesteps the significant metal losses often encountered in traditional methods that necessitate initial pH adjustments. The NF270 and DK membranes were characterized and evaluated. DK demonstrated superior rejection capabilities for high-valence metal ions with over 99.0 % rejection, while allowing a notable 40.1 % lithium passage rate. Furthermore, DK showcased enhanced acid resistance compared to NF270. The findings underscore the characteristics essential for NF membranes in this application, and the study affirms the feasibility of membrane-based lithium recovery, marking a stride in battery recycling research.

Modelisation of the Biomethane Accumulation in Anaerobic Co-Digestion of Whey and Sugarcane Molasse Mixtures

The biomethane accumulation of several combinations of whey and sugarcane molasses, inoculated with sludge from a treatment facility of one of the dairy enterprises of the Imbabura province in Ecuador, was assessed in the current experiment at a constant COD0/VSin ratio of 0.5. The whey/molasses (W:M) ratios for each treatment were (in % (m/m)) 0:100, 25:75, 50:50, 75:25, and 100:0, with a constant temperature of 37 °C and an initial pH adjustment of 7.5. Half a litre of total mixes was used for each treatment in duplicate. Six kinetic models were evaluated to account biomethane accumulation in anaerobic co-digestion processes in batch of whey and sugarcane molasses. Five of these have been tested by other researchers, and one was developed by modifying a first-order model to consider changes in the biomethane accumulation profile. This proposed model, along with the modified two-phase Gompertz model, resulted in the ones that were best able to adjust the experimental data, obtaining in all cases an R2 ≥ 0.949, indicating the accuracy of both models. In addition, the proposed here model has five parameters, one less than the modified two-phase Gompertz model, making it more straightforward and robust.

Enhanced anaerobic digestion of waste activated sludge by alkaline protease-catalyzing hydrolysis: Role and significance of initial pH adjustment

Anaerobic digestion has been regarded as a preferred technology for carbon/energy recovery from waste activated sludge via volatile fatty acids (VFAs) and methane production. Although the enzyme pretreatment has been investigated, specific roles of initial pH adjustment on alkaline protease (AP) catalysis and its influence on anaerobic digestion have been rarely reported. This work demonstrated that the increasing initial pH displayed positive mutual promotion of AP-catalyzing hydrolysis for facilitating sludge solid reduction and biodegradable organic matter solubilization. The release of carbon source and phosphorus were also promoted, while the soluble NH4+-N content was decreased at increasing alkaline conditions, which was beneficial for hydrolysate bio-utilization. Nevertheless, initial pH = 10 was the optimum condition for maximizing VFAs and methane recovery after AP-catalyzing sludge hydrolysis. Under such condition, the sludge solid reduction and sludge hydrolysis rate constants were 0.7981 h−1 and 1.1288 h−1, respectively, which were 6.40 and 2.45 times higher than those in the control. Meanwhile, the biochemical acidogenic potential of 447.0 mg COD VFAs/g VSS and the estimated biochemical methane potential of 437.6 mL CH4/g VSS were achievable. The identification of the mutual promotion distribution revealed that initial pH adjustment presented more close association with anaerobic digestion than AP catalysis and its contribution was amplified with increasing pH. Apparently, such AP catalysis with initial pH adjustment is a promising technology for sustainable sludge management with economic benefits and carbon-emission reduction.

Taguchi method in the optimization of municipal wastewater treatment by electrocoagulation integrated with zeolite

Electrocoagulation integrated with zeolite in a single step was efficiently applied to the treatment of municipal wastewater with saline water infiltrate. In the first part, the influence of the initial pH values of the solutions and the addition of natural zeolite and NaCl electrolyte on the removal efficiency was investigated. Decreasing of electrical conductivity (17.00–65.79%), turbidity (61.28–97.44%), chemical oxygen demand (77.08–92.71%) and total Kjeldahl nitrogen (33.33–90.48%) was obtained. The initial pH of the wastewater, the addition of zeolite and NaCl electrolyte shows a significant influence on the turbidity reduction, total Kjeldahl nitrogen removal, electrode consumption, and settling properties, while its effects on chemical oxygen demand and electrical conductivity reduction is less pronounced. An increase in solution temperature is higher in an acidic (pH = 4) medium and is related to higher electrode consumption. In the second part, Taguchi’s L9 orthogonal array method was used to optimize​ municipal wastewater treatment by electrocoagulation and zeolite addition. The effects of initial pH values (pH = 9, pH = 4, and without initial pH adjustment), zeolite addition (natural, synthetic and without), electrolyte addition (NaCl and Na2SO4 and without), and experiment duration time (15, 30 and 45 min) on chemical oxygen demand and turbidity were examined. The results of Taguchi optimization revealed that the most effective factor for the chemical oxygen demand removal is zeolite addition, while the initial pH is for turbidity. Optimal conditions for chemical oxygen demand and turbidity are: the initial pH = 4, without adding electrolyte and an experimental duration of 15 min. However, the addition of zeolite has a dissimilar effect on chemical oxygen demand and turbidity. For chemical oxygen demand, better results are obtained using synthetic zeolite and for turbidity without any type of zeolite.

Effects of varied culture conditions on crude bacitracin produced by Bacillus subtilis isolated from the soil

Aim: To determine the effects subjecting Bacillus subtilis to different cultural conditions will have on the crude Bacitracin produced in inhibiting the growth of known susceptible organisms. Method: Bacillus subtilis known to be a bacitracin producer was isolated from the soil and identified. The isolate was used to produce bacitracin under various culture conditions with peptone water serving as a basal fermentation medium. The resultant crude bacitracin produced was checked for its antimicrobial activity against Staphylococcus aureus by using the agar well diffusion method. Culture conditions varied were the addition of glycerol, arabinose, fructose, mannose, sucrose, urea, MgSO4, CaCO3, KCl, sodium citrate, and the adjustment of initial pH at pH 5, 6, 8 and 9. Bacitracin production was done at 37 oC for 4 days in an orbital shaker incubator rotating at 150rpm. Result: The antimicrobial activity against the test organism (Staphylococcus aureus) was determined by the mm of the zone of inhibition, with the best result obtained from the addition of glycerol (14 mm) and the least zone of inhibition was observed from the addition of MgSO4 (3 mm). Also, pH 8 produced the best result (20 mm), while at pH 5, the antimicrobial activity of the crude bacitracin was reduced (14 mm). Conclusion: Bacitracin produced from Bacillus subtilis under various culture conditions has antimicrobial effect against Staphylococcus aureus. The highest antimicrobial activities can be gotten by the addition of glycerol and by raising the initial pH of the basal broth to pH 8 while that produced by addition of MgSO4 will have the lowest antimicrobial effect.

Hydrogen Production from Enzymatic Hydrolysates of Alkali Pre-Treated Giant Reed (Arundo donax L.)

The perennial rhizomatous grass giant reed (Arundo donax L.) can be exploited to produce hydrogen by dark fermentation. This implies a high availability of simple sugars, like glucose and xylose, and, thus, a pre-treatment is necessary to remove lignin and expose the holocellulose to enzymatic attack. This study aimed at evaluating the hydrogen production from giant reed hydrolysates. Giant reed dry meal was pre-treated with diluted NaOH (1.2% weight/weight), then the solid fraction was separated from the alkaline black liquor by filtration, enzymatically hydrolyzed with a cellulase blend (Cellic CTec2), and fermented in mesophilic batch conditions with a microbial consortium derived from pig slurry. The impact on hydrogen yield of initial pH was evaluated by comparing the hydrogen production from hydrolysates with not adjusted (5.3) or adjusted initial pH (8.7) using NaOH or alkaline black liquor. The highest hydrogen yield, 2.0 mol/mol of hexoses, was obtained with alkaline initial pH 8.7, regardless of how the pH adjustment was managed. The yield was 39% higher than that obtained in reactors with initial pH 5.3. In conclusion, thermo-alkaline pre-treatment followed by enzymatic saccharification and initial pH adjustment at 8.7 with the black liquor remaining after pre-treatment is a promising strategy to produce hydrogen from giant reeds in dark fermentation.

PH buffering of nutrient solutions with differing water quality in small-scale hydroponic systems

Nutrient solution pH and water quality are challenges faced by small-scale hydroponic growers. The objective was to evaluate the use of the pH buffer 2-(N-morpholino) ethanesulfonic acid (MES) as an initial dose to simplify pH management in small-scale hydroponic systems, and its effectiveness within the range of alkalinity that commonly occurs in irrigation water. Two experiments tested pH buffering capacity in response to acid-base titrations, and when growing compact tomatoes (Lycopersicon esculentum) over a nine-week cycle. MES was applied at 0 to 10 mM in water and nutrient solutions at five water qualities ranging from 4 to 295 mg.L−1 CaCO3 alkalinity. Solution buffering increased with addition of fertilizer, increasing doses of MES, and increasing water alkalinity. The MES decreased pH variability over time in all water qualities, without affecting plant growth or causing nutritional disorders. The pH of solutions without MES ranged from 4.5 to 8.0 over the growing cycle, whereas in 10 mM MES solutions it ranged from 5.7 to 7.0 for alkalinities 0 to 146 mg.L−1 CaCO3 and reached 7.6 for 295 mg.L−1 CaCO3. Titration of nutrient solutions after tomato growth indicates that MES acid-buffering capacity was partially consumed along the trial, which was probably the result of MES decomposition or interaction with acids and bases in solution. Results indicate that a single 10 mM MES application could be used by small-scale hydroponic growers to provide acceptable pH buffering for low to moderate alkalinity levels, but initial pH adjustment may be necessary and solution pH may still increase with high alkalinity.

Electrocoagulation vs. Integrate Electrocoagulation-Natural Zeolite for Treatment of Biowaste Compost Leachate—Whether the Optimum Is Truly Optimal

Natural zeolites are well-known materials widely applied in the environmental remediation treatment process. However, the integration of various treatment methods is exceedingly investigated for achieving satisfactory effluent quality. In this paper, the integration of electrocoagulation and natural zeolite was evaluated in the treatment of biowaste compost leachate in a single step. The influence of different distances of electrodes (1.5, 3, and 4.5 cm), stirring speed (70, 200, and 400 rpm), the addition of natural zeolite and electrolyte NaCl on the efficiency of treatment of biowaste compost leachate has been carried out. Process efficiency was evaluated by measuring the change of pH value, electrical conductivity, temperature, turbidity, chemical oxygen demand (COD), total Kjeldahl nitrogen (TNK), total solids, and sludge settling test. The Taguchi method was applied to optimize biowaste compost leachate treatment. Experiments are planned according to Taguchi’s L8 (24 41) orthogonal array. The stirring speed, electrode distance, electrolyte and zeolite addition, solution initial pH adjustment were chosen as controllable factors, and their impact on COD, turbidity, TNK, settling rate, and electrode consumption were studied. Results show that optimal conditions depend on the parameter of interest and that optimal values for a particular parameter are not always the optimum if the desired goal is considered.

Treatment of a poultry slaughterhouse wastewater using advanced oxidation processes

In the present study, different chemical species such as hydrogen peroxide, potassium persulfate, titanium dioxide, and iron salts were employed for the treatment of a real poultry slaughterhouse wastewater (poultry-SWW) under UV-C light (254 nm). The experiments lasted 150 min and were carried out in a batch photoreactor aiming at investigating the most efficient operating conditions in terms of total organic carbon (TOC) removal. UV photolysis or Fe-doped TiO2 photocatalysis were practically inefficient, while application of un-doped TiO2 resulted in 44% TOC removal after 60 min of UV irradiation. The use of 98 mM H2O2 under UV light led to 74% TOC removal after 150 min. The addition of Fe(II) ions to this system coupled with initial pH adjustment to 3 increased the TOC removal to 82.5% after 150 min. Finally, the combination of UV with 15 mM of K2S2O8 resulted in 85% TOC removal. This was the first time when the persulfate oxidation was successfully applied for SWW treatment. To assess the energy efficiency of the various photochemical treatment processes tested, estimation of the electric energy per order (EEO, kWh/m3/order) was conducted. The lowest EEO value of 226 kWh/m3/order was observed for the UV/K2S2O8 process and compared with other photochemical treatment technologies. The effluent quality of UV/K2S2O8 treatment complies with the discharge water quality standards of the developed countries. Therefore, it is possible to use UV/K2S2O8 process as a post-treatment after biological treatment of SWW to meet the discharge water standards and reuse the wastewater.

Comparison of Initial pH Adjustment Prior to Thermophilic Anaerobic Digestion of Lime-Treated Corn Stover via Liquid Digestate or CO2

Neutralization with liquid digestate and CO2 was compared herein to adjust the pH of lime-treated corn stover. The effects on the thermophilic (55 °C) anaerobic digestion were also analyzed. Liquid digestate neutralization (LDN) caused a decrease in pH from 10.5 to 7.5 in 60 h and accumulation of acetic/isobutyric acids. The CO2 neutralization (CN) under solid-state conditions reduced the pH from 10.5 to 8.5 in 30 min, which is faster than that of LDN and did not affect the subsequent anaerobic digestion. Biomethane production rate indicates that LDN contributed to the performance of anaerobic digestion, but this was not sufficient to compensate for the loss of total biomethane yield, resulting in a negative net profit (i.e., revenue from increased energy production minus reagent cost). For CN under solid-state conditions, the biomethane production was highest in both liquid- and solid-state anaerobic digestion, and also obtained a net profit of 98.74–100.89 RMB/tonne dry biomass. Therefore, the solid-state condition CN is a more efficient and economic method for adjusting initial pH of lime-treated corn stover.

Production of Fermented Soymilk Drink Containing Probiotic Bacillus coagulans

The aim of this study is to produce a fermented soymilk drink using Bacillus coagulans. This was performed in the Microbiology laboratory of Nnamdi Azikiwe, University. 20ml aliquots of soymilk containing Bacillus coagulans was fermented at 28°C, 37°C, 42°C and 50°C for period of 9 h. The pH of the soymilk and growth of Bacillus coagulans was checked during the fermentation period. The effect of sugar supplementation and adjustment of initial pH on soymilk fermentation was also checked. A 9-point hedonic scale was used by the sensory panelist for the sensory evaluation of the fermented soymilk. At 28°C, pH of soymilk did not decrease and cell count did not increase throughout the fermentation period. Fermentation at 37°C, 42°C and 50°C recorded decrease in pH and increase in cell count. Addition of 0.5% sucrose improved acid production and maintained a good cell count. Concentrations above 0.5% sucrose saw a slight decline in cell count. Glucose concentration of 0.5% to 2% improved acid production. Glucose concentration of 0.1% to 1% improved the growth of the probiotic cells. Concentration above 1% caused a drop in probiotic cell count. Adjustment of soymilks initial pH and addition of 0.5% glucose resulted in pH drop to 4.5 after 9h fermentation at 50°C. The fermented soymilk had moderate overall acceptability by the sensory panelist. Bacillus coagulans can be used as probiotic of choice to produce a fermented soymilk.

Influence of volatile solids and pH for the production of volatile fatty acids: Batch fermentation tests using sewage sludge

The aim of this work was to study the effect of volatile suspended solid (VSS) and pH on volatile fatty acids (VFA) production from waste activated sludge (WAS) fermentation by means of batch tests. The final goal was to gain insights to enhance VFA stream quality, with the novelty of using WAS with high sludge retention time. Results revealed that the optimum conditions to maximize VFAs and minimize nutrients and non-VFA sCOD are a VSS concentration of 5.9 g/L and initial pH adjustment to pH 10. The WAS bacterial community structures were analysed according to Next Generation Sequencing (NGS) of 16S rDNA amplicons. The results revealed changes of bacterial phyla abundance in comparison with the batch test starting condition.

Effect of Organic Substances on Nutrients Recovery by Struvite Electrochemical Precipitation from Synthetic Anaerobically Treated Swine Wastewater.

Anaerobically treated swine wastewater contains large amounts of orthophosphate phosphorus, ammonium nitrogen and organic substances with potential nutrients recovery via struvite electrochemical precipitation post-treatment. Lab-scale batch experiments were systematically conducted in this study to investigate the effects of initial pH, current density, organic substances upon nutrients removal, and precipitates quality (characterized by X-ray diffraction, scanning electron microscopy and element analysis via acid dissolution method) during the struvite electrochemical precipitation process. The optimal conditions for the initial pH of 7.0 and current density of 4 mA/cm2 favoured nutrients removal and precipitates quality (struvite purity of up to 94.2%) in the absence of organic substances. By contrast, a more adverse effect on nutrients removal, morphology and purity of precipitates was found by humic acid than by sodium alginate and bovine albumin in the individual presence of organic substances. Low concentration combination of bovine albumin, sodium alginate, and humic acid showed antagonistic inhibition effects, whereas a high concentration combination showed the accelerating inhibition effects. Initial pH adjustment from 7 to 8 could effectively mitigate the adverse effects on struvite electrochemical precipitation under high concentration combined with organic substances (500 mg/L bovine albumin, 500 mg/L sodium alginate, and 1500 mg/L humic acid); this may help improve struvite electrochemical precipitation technology in practical application for nutrients recovery from anaerobically treated swine wastewater.

Anaerobic digestion of dissolved air floatation slurries: Effect of substrate concentration and pH

Dissolved air floatation (DAF) slurries are an unconventional substrate for anaerobic digestion owing to their high COD concentrations, mainly consisting of lipids. Batch anaerobic digestion of DAF slurry was conducted to investigate the methane production through dilution of the slurry (with mineral salt medium, MSM) and by adjusting the initial pH (to 6.0, 5.5, 5.0 and 4.5) The performance of two different inocula, namely waste activated sludge (WAS) treating domestic wastewater and anaerobic granular sludge (AGS) treating dairy wastewater, was compared. The DAF slurry had a pH of 7.03 ( ± 0.07), conductivity of 6.69 ( ± 0.54) mS/cm, and a COD and ammonium concentration of 85.96 ( ± 8.32) g/L and 168.49 ( ± 1.79) mg N/L, respectively. DAF slurry without dilution produced a maximum methane yield of 58.60 ( ± 1.97) mL/g COD after 30 days of incubation. Dilution with MSM up to three folds, improved the methane yield by about 67%–105% and 67%–177% with WAS and AGS as the inoculum, respectively. Higher methane productivity was achieved at an inoculum to substrate ratio of at least 1.5 for WAS and 2.0 for AGS, respectively. AGS was more effective with a 20% increase in methane production from 3-fold diluted DAF slurry when compared with WAS. Adjustment of the initial pH of the DAF slurry to 6.0 resulted in a methane yield of 359.71 ( ± 32.72) mL/g COD, which was 23% higher than the control without pH adjustment. This study demonstrated that the methane yield from DAF slurries can be enhanced through substrate dilution and initial pH adjustment. • Methane yield enhanced by 23% upon adjustment of initial pH of the DAF slurry to 6.0. • 3-fold dilution increased methane production rate by 275% with granular sludge. • Anaerobic granular sludge is an efficient inoculum for methanogenesis of DAF slurry. • Lag phase duration was not shortened by dilution or pH adjustment of the DAF slurry. • Optimal inoculum to substrate ratio at least 1.5 and 2.0 for WAS and AGS, respectively.

Development and Validation of a Method for Ionic Dyes Determination in Guava Fibrous Matrix Using Reverse Phase Chromatography

In this work, a method for food dyes determination in fibrous fruit pulps was developed. The analytes Tartrazine, Twilight Yellow FCF, Erythrosine, Orange G and Allura Red were analyzed by reverse phase chromatography with diode array (DAD) detection. The sample preparation consisted of a simple procedure, based on initial pH adjustment of the sample (pH = 10.0), followed by an extraction with acetonitrile (5.0 mL and agitation for 30 s), ultrasonic (5 min) and centrifugation (4000 rpm, 20 ºC, for 20 min). The supernatant had its pH adjusted to 3.0, followed by filtration. The method was validated, with the analytical curves prepared in a matrix, in the range of 50 to 150 mg kg-1 for all the analytes studied (determination coefficients (R2) > 0.99). The method veracity was evaluated in terms of recovery, obtaining recoveries of 93-110%. The repeatability was less than 13.7%, while the intermediate precision was less than 19.0%. The limits of detection and quantification were determined, being 15 and 30 mg kg-1, respectively. The combined relative uncertainties were less than 13.8%. Therefore, it can be concluded that a simple method was developed, suitable for monitoring the presence of dyes in fruit pulps in routine analyzes.

Electrochemical Removal of Copper Ion Using Coconut Shell Activated Carbon

In this work, coconut shell activated carbon (CSAC) electrode was evaluated to remove copper ion via electrochemical processes. CSAC electrode and graphite were applied as the cathode and the anode, respectively. The reusability of the electrode, the effects of initial pH, applied voltage and initial concentration were studied. The electrochemical process was carried out for 3 h of treatment time, and the electrodes (anode and cathode) were separated by 1 cm. The results revealed that CSAC is proven as a reusable electrode to remove copper ion, up to 99% of removal efficiency from an initial concentration of 50 ppm after it had been used three times. From the observation, the removal efficiency was optimum at an initial pH of 4.33 (without any initial pH adjustment). The applied voltage at 8 V showed a higher removal efficiency of copper ion compared to at 5 V.

Optimization of an electrocoagulation-flotation system for domestic wastewater treatment and reuse

ABSTRACT The risks inherent to the inadequate domestic wastewater disposal, allied to the water growing demand, scarcity, and pollution problems, have highlighted the importance of adopting treatment techniques that not only target the sewage discharge, but also its reuse. For this reason, the objective of this study was to evaluate the best conditions of an electrocoagulation-flotation system for domestic wastewater treatment and urban reuse. To achieve this, an effects study followed by two rotatable central composite experimental designs 2² was performed, considering: agitation, electrical current, electrolysis time, inter-electrodes distance, and initial pH. The electrocoagulation-flotation system was composed of a cylindrical acrylic reactor with a working volume of 1 L, with two aluminium electrodes connected to a direct-current power supply. Results showed that electrical current and electrolysis time were the most influent operational parameters on domestic wastewater treatment in the electrocoagulation-flotation system. The initial pH adjustment was also important due the pH increase tendency observed in the results. The best conditions of agitation, inter-electrodes distance, electrolysis time, electrical current, and initial pH for domestic wastewater treatment and urban reuse were 262.5 rpm, 1 cm, 25 min, 1.65 A, and 6, respectively. Under these conditions, turbidity and colour removals higher than 98% and 92% were reached respectively, as well as residual turbidity lower than 6 NTU and final pH of 8 were achieved, following the Brazilian standards and guidelines for urban reuse. Thus, the electrocoagulation-flotation system studied was effective for domestic wastewater treatment and reuse for urban supply purposes.

Enhancing efficiency and economics of phosphorus recovery process by customizing the product based on sidestream characteristics - an alternative phosphorus recovery strategy.

The enhanced biological phosphorus removal process makes the phosphorus recovery feasible from the dewatering streams of biological sludge. The physicochemical properties of these sidestreams, as an input to a crystallizer, are different before and after anaerobic digestion. In this study, phosphorus recovery by calcium phosphate is proposed for pre-digestion sidestreams and by struvite precipitation for post-digestion sidestreams. The thermodynamic modeling followed by experimental tests was performed to evaluate the recovery efficiency and product properties of struvite and calcium phosphates. The variations in phosphorus recovery potential, reaction kinetics and particle size distribution emphasize the importance of the adjustment of initial supersaturation and pH of the reaction. The optimum pH, considering the economics and recovery efficiency, for both calcium phosphate and struvite precipitation was found to be pH = 8.5, whereas further increase of pH will not improve the overall efficiency of the process. In the case of calcium phosphate precipitation, it was shown that possible phase transformations should be considered and controlled as they affect both process efficiency and product properties. The economic evaluation indicated that the optimized operational condition should be determined for the phosphorus recovery process and that chemical costs for the production of calcium phosphates is lower than for struvite.

Hybridization of Nanodiamond and CuFe-LDH as Heterogeneous Photoactivator for Visible-Light Driven Photo-Fenton Reaction: Photocatalytic Activity and Mechanism

Establishing a heterojunction for two kinds of semiconductor catalysts is a promising way to enhance photocatalytic activity. In this study, nanodiamond (ND) and CuFe-layered double hydroxide (LDH) were hybridized by a simple coprecipitation method as a novel heterojunction to photoactivate H2O2. The ND/LDH possessed a hydrotalcite-like structure, large specific surface area (SBET = 99.16 m2/g), strong absorption of visible-light and low band gap (Eg = 0.94 eV). Under the conditions of ND/LDH dosage 0.0667 g/L, H2O2 concentration 19.6 mmol/L, and without initial pH adjustment, 93.5% of 10 mg/L methylene blue (MB) was degraded within 120 min, while only 78.3% of MB was degraded in the presence of LDH instead of ND/LDH. The ND/LDH exhibited excellent stability and maintained relatively high activity, sufficient to photoactivate H2O2 even after five recycles. The mechanism study revealed that in the heterojunction of ND/LDH, the photoelectrons transferred from the valence band of LDH (Cu/Fe 3d t2g) to the conduction band of LDH (Cu/Fe 3d eg) could spontaneously migrate onto the conduction band of ND, promoting the separation of photo-induced charges. Thus, the photoelectrons had sufficient time to accelerate the redox cycles of Cu3+/Cu2+ and Fe3+/Fe2+ to photoactivate H2O2 to produce hydroxyl radicals, resulting in excellent photo-Fenton efficiency on MB degradation.