Study on the performance and mechanism of cobaltous ion removal from water by a high-efficiency strontium-doped hydroxyapatite adsorbent.

In this study, a high-efficiency strontium-doped hydroxyapatite (Sr-HAP) adsorbent was synthesized by a sol-gel method for removing cobaltous ions (Co(II)) from water. The effects of adsorbent dose, initial solution pH, initial Co(II) concentration and temperature on the removal performance of Co(II) were investigated. Experimental results indicated that the optimum Sr-HAP dose was 0.30 g/50 mL solution, the Sr-HAP adsorbent could effectively remove Co(II) in a wide pH range of 3-8. Increasing temperature was conducive to the adsorption, and the maximum Co(II) adsorption capacity by Sr-HAP reached 48.467 mg/g at 45 °C. The adsorption of Co(II) followed the pseudo-second-order kinetic model, indicating that the Co(II) adsorption by Sr-HAP was attributed mainly to chemisorption. The isothermal adsorption results showed that at lower Co(II) equilibrium concentration, the Langmuir model fitted the data better than the Freundlich model but opposite at higher Co(II) equilibrium concentration. Therefore, the adsorption of Co(II) was a process from monolayer adsorption to multilayer adsorption with the increase of the Co(II) equilibrium concentration. The diffusion analysis of Co(II) to Sr-HAP indicated that the internal diffusion and surface adsorption were the rate-controlled steps of Co(II) adsorption. Thermodynamic study demonstrated that the Co(II) adsorption process was spontaneous and endothermic. The mechanism study revealed that in addition to chemisorption, Sr-HAP also removed Co(II) ions from water via ion exchange and surface complexation.

[Preparation of Magnetic Iron Oxide/Mulberry Stem Biochar and Its Effects on Dissolved Organic Carbon and Arsenic Speciation in Arsenic-Contaminated Soils].

In this study, original mulberry-biochar (M-BC) and magnetic iron oxide/mulberry stem biochar (Fe-BC) materials were prepared and characterized using mulberry stems as the raw material. The effects of carbonized temperature of Fe-BC and M-BC on dissolved organic carbon (DOC) and arsenic(As) speciation in soil leaching solutions were studied using soil incubation experiments. The results showed that:① Fe-BC was mainly composed of Fe3O4 and was magnetic, and the main functional groups were a C＝O double bond, O-H bond, C-O bond, and Fe-O bond. The point of zero charge values (pHzpc) of Fe-BC-400, Fe-BC-500, and Fe-BC-600 were 8.92, 8.74, and 9.19, respectively, and the specific surface areas of Fe-BC-400, Fe-BC-500, and Fe-BC-600 were 447.412, 482.697, and 525.708 m2·g-1, respectively. ② With the increase in the carbonization temperature of M-BC and Fe-BC, the ρ(DOC) of soil leaching solution decreased 11.6-315.6 mg·L-1 and 78-365.6 mg·L-1, respectively. The DOC concentration of soil leaching solution was negatively correlated with soil EC. On day 35 of the incubation experiments, compared with that in soil after incubation without biochar (control), the As concentration of the soil leaching solution with Fe-BC-600 decreased by 55.96%, and there was no significant correlation between the As concentration of the soil leaching solution and the DOC concentration of the soil. ③ The available As concentration on day 35 in soil after incubation with Fe-BC was lower than that of the control group; the available As concentration on day 35 in soil incubated with Fe-BC-600 was reduced by 39.21%. ④ The residue As concentration on day 35 in soil incubated with M-BC decreased by 17.76%-49.11%. The residue As content on day 35 in soil incubated with Fe-BC-600 increased by 80%. Fe-BC-600 was most beneficial to reduce the DOC concentration and the available As content in soil leaching solution and increased the residue As content, thus reducing the bioavailability of soil arsenic. Therefore, this study can provide a theoretical basis for magnetic iron oxide/biochar remediation in arsenic-contaminated soil.

Integration of Zeolite Membrane Bioreactor With Granular Sludge-Based Anammox in High-Efficiency Nitrogen Removal From Iron Oxide Red Wastewater.

Acquisition of stable nitritation and efficient anammox play a crucial role in partial nitritation (PN) combined with anammox for nitrogen removal from ammonium-rich wastewater. Due to the limitation of ammonia-oxidizing bacteria (AOB) enrichment and nitrite-oxidizing bacteria (NOB) control in traditional membrane biological reactor (MBR), it can result in a lower nitrite production rate (NPR) and unstable PN, eventually reducing the nitrogen removal rate (NRR) via PN-anammox. In this study, we developed a zeolite membrane biological reactor (ZMBR) to enhance the PN of iron oxide red wastewater (IORW), in which the biofilm derived from the zeolite surface can provide free ammonia (FA)-containing microenvironment for AOB enrichment and NOB inhibition. The results showed that ZMBR can tolerate a higher influent nitrogen loading rate (NLR) of 2.78 kg/(m3⋅day) in comparison to the traditional MBR [2.02 kg/(m3⋅day)] and the NPR in ZMBR and traditional MBR were 1.39 and 0.96 kg/(m3⋅day), respectively. The mass concentration ratio of NO 2 - -N/ NH 4 + -N ranged from 1.05 to 1.33 in ZMBR, suggesting a suitable condition for nitrogen removal via anammox. Subsequently, the domesticated granular sludge obtained from a paper-making wastewater treatment was used as the carrier of anammox bacteria to remove nitrogen. After 93 days of operation, the NRR was observed to be 2.33 kg/(m3⋅day) and high-throughput sequencing indicated that the relatively higher abundance (45.0%) of Candidatus Kuenenia stuttgartiensis was detected in the granular sludge of the bottom part of the reactor, which can produce more proteins and lipids, suggesting a good settleability. Overall, this study provides a high-efficient method to control PN and domesticate anammox for nitrogen removal from IORW.

Effective Remediation of Arsenic-Contaminated Soils by EK-PRB of Fe/Mn/C-LDH: Performance, Characteristics, and Mechanism.

Arsenic is highly toxic and carcinogenic. The aim of the present work is to develop a good remediation technique for arsenic-contaminated soils. Here, a novel remediation technique by coupling electrokinetics (EK) with the permeable reactive barriers (PRB) of Fe/Mn/C-LDH composite was applied for the remediation of arsenic-contaminated soils. The influences of electric field strength, PRB position, moisture content and PRB filler type on the removal rate of arsenic from the contaminated soils were studied. The Fe/Mn/C-LDH filler synthesized by using bamboo as a template retained the porous characteristics of the original bamboo, and the mass percentage of Fe and Mn elements was 37.85%. The setting of PRB of Fe/Mn/C-LDH placed in the middle was a feasible option, with the maximum and average soil leaching toxicity removal rates of 95.71% and 88.03%, respectively. When the electric field strength was 2 V/cm, both the arsenic removal rate and economic aspects were optimal. The maximum and average soil leaching toxicity removal rates were similar to 98.40% and 84.49% of 3 V/cm, respectively. Besides, the soil moisture content had negligible effect on the removal of arsenic but slight effect on leaching toxicity. The best leaching toxicity removal rate was achieved when the soil moisture content was 35%, neither higher nor lower moisture content in the range of 25-45% was conducive to the improvement of leaching toxicity removal rate. The results showed that the EK-PRB technique could effectively remove arsenic from the contaminated soils. Characterizations of Fe/Mn/C-LDH indicated that the electrostatic adsorption, ion exchange, and surface functional group complexation were the primary ways to remove arsenic.

Mn-Fe Layered Double Hydroxide Intercalated with Ethylene-Diaminetetraacetate Anion: Synthesis and Removal of As(III) from Aqueous Solution around pH 2-11.

A novel adsorbent Mn-Fe layered double hydroxides intercalated with ethylenediaminete-traacetic (EDTA@MF-LDHs) was synthesized by a low saturation coprecipitation method. The behavior and mechanism of As(III) removed by EDTA@MF-LDHs were investigated in detail in comparison with the carbonate intercalated Mn-Fe layered double hydroxides (CO3@MF-LDHs). The results showed that EDTA@MF-LDHs had a higher removal efficiency for As(III) than As(V) with a broader pH range than CO3@MF-LDH. The large adsorption capacity of EDTA@MF-LDHs is related to its large interlayer spacing and the high affinity of its surface hydroxyl groups. The maximum adsorption capacity for As(III) is 66.76 mg/g at pH 7. The FT-IR and XPS characterization indicated that the removal mechanism of the As(III) on EDTA@MF-LDHs include surface complexation, redox, and ion exchange.

Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd5(PO4)3OH] at 25-45°C.

BACKGROUND: The substitution of Ca(2+) in Ca-hydroxylapatite by toxic Cd(2+) can cause the forming of Cd-hydroxylapatite and is a significant issue in a great variety of research areas, which hence needs an understanding of the essential physicochemical characteristics. Unfortunately, the solubility product and thermodynamic data for Cd-hydroxylapatite in water under a variety of conditions now are lacking. Little information has been reported by previous researchers. Additionally, the dissolution mechanism of Cd-hydroxylapatite has never been studied. RESULTS: Dissolution of the synthetic cadmium hydroxylapatite [Cd-HAP, Cd5(PO4)3OH] in HNO3 solution (pH = 2), ultrapure water (pH = 5.6) and NaOH solution (pH = 9) was experimentally studied at 25, 35 and 45°C. Characterization by XRD, FT-IR and FE-SEM proved that Cd-HAP solids showed no recognizable change during dissolution. For the Cd-HAP dissolution in aqueous acidic media at initial pH 2 and 25°C, the solution cadmium and phosphate concentrations increased rapidly and reached the peak values after 20-30 days and 10 days reaction, respectively. Thereafter, the Cd-HAP dissolution rate decreased slowly, whereas the solution Cd/P molar ratio increased constantly from 1.65-1.69 to 6.61-6.76. The mean K sp values for Cd5(PO4)3OH were determined to be 10(-64.62) (10(-64.53)-10(-64.71)) at 25°C, 10(-65.58) (10(-65.31)-10(-65.80)) at 35°C and 10(-66.57) (10(-66.24)-10(-66.90)) at 45°C. Based on the obtained solubility data from the dissolution at initial pH 2 and 25°C, the Gibbs free energy of Cd5(PO4)3OH forming [Formula: see text] was determined to be -3,970.47 kJ/mol (-3,969.92 to -3,970.96 kJ/mol). Thermodynamic parameters, ΔG (0), ΔH (0), ΔS (0), and [Formula: see text] for the dissolution process of Cd-HAP in aqueous acidic media at initial pH 2 and 25°C were calculated 368,710.12 J/K mol, -158,809.54 J/mol, -1,770.20 and -869.53 J/K mol, respectively. CONCLUSIONS: Based on the experimental results of the present work and some previous researches, the cadmium hydroxylapatite (Cd-HAP) dissolution in aqueous media is considered to have the following coincident processes: the stoichiometric dissolution coupled with protonation and complexation reactions, the non-stoichiometric dissolution with Cd(2+) release and PO4 (3-) sorption and the sorption of Cd(2+) and PO4 (3-) species from solution backwards onto Cd-HAP surface. The obtained solubility products (K sp) 10(-64.62) (10(-64.53)-10(-64.71)) for Cd-HAP was approximately 7.62-5.62 log units lower than 10(-57)-10(-59) for calcium hydroxylapatite (Ca-HAP).Graphical abstractDissolution of cadmium hydroxylapatite [Cd5(PO4)3OH].