Yunfeng Xu

Removal efficiency of arsenate and phosphate from aqueous solution using layered double hydroxide materials: intercalation vs. precipitation

Adsorption behaviours of arsenate and phosphate over Mg-based and Ca-based layered double hydroxide (LDH) adsorbents have been examined in kinetics and thermodynamics. Removal of these anions from aqueous solution follows the Lagergren first-order and/or pseudo-second-order model, and the adsorption isotherm is well fitted with either the Langmuir or the Freundlich model. Structure analysis of used LDH adsorbents reveals that two processes, i.e. intercalation and precipitation, are responsible for the anion removal. Adsorption over the Mg-based LDH adsorbent occurs by way of intercalation into the interlayer spacing while that over the Ca-based adsorbent occurs by means of precipitation of dissolved Ca2+ with the anion. More particularly, we have found that As(V) at a concentration below 10 mg/L can be very efficiently removed through intercalation into the interlayer of reconstructed MgAl-LDHs, with less than 0.010 mg/L of As left in solution. We have also noted that phosphate at [P] up to 100 mg/L can be quickly and effectively removed through precipitation with CaAl-Cl-LDH, giving rise to ∼0.1 mg/L of P left in solution with the maximum adsorption amount up to 135 mg/g. Therefore, these two LDH materials (calcined Mg3Al-CO3-LDH and uncalcined Ca2Al-Cl-LDH) are potential cost-effective adsorbents for arsenate and phosphate, respectively.

Effective removal and fixation of Cr(VI) from aqueous solution with Friedel's salt.

Friedels salt (3CaO x Al2O3 x CaCl2 x 10 H2O or Ca4Al2(OH)12Cl2(H2O)4) is a calcium aluminate hydrate formed by hydrating cement or concrete in seawater at a low cost. In the current study, we carefully examined the adsorption behaviors of Friedels salt for Cr(VI) from aqueous solution at different concentrations and various initial pHs. The adsorption kinetic data are well fitted with the pseudo-first-order Lageren equation at the initial Cr(VI) concentration from 0.10 to 8.00 mM. Both the experimental and modeled data indicate that Friedels salt can adsorb a large amount of Cr(VI) (up to 1.4 mmol Cr(VI)/g) very quickly (t1/2 = 2-3 min) with a very high efficiency (>99% Cr(VI) removal at [Cr] < 4.00 mM with 4.00 g/L of adsorbent) in the pH range of 4-10. In particular, the competitive adsorption tests show that the Cr(VI) removal efficiency is only slightly affected by the co-existence of Cl(-) and HCO3(-). The Cr(VI)-fixation stability tests show that only less than 0.2% adsorbed Cr(VI) is leaching out in water at pH 4-10 for 24 h because the adsorption/exchange of Cr(VI) with Friedels salt leads to the formation of a new stable phase (3CaO x Al2O3 x CaCrO4 x 10 H2O). This research thus suggests that Friedels salt is a potential cost-effective adsorbent for Cr(VI) removal in wastewater treatment.

Enhanced removal of triphosphate by MgCaFe-Cl-LDH: Synergism of precipitation with intercalation and surface uptake

Triphosphate (TPP) is an important form of phosphate pollutants while its removal investigation has been just started now. This research examined the removal of triphosphate using Mg(2-x)Ca(x)FeCl-LDH (x = 0-2) as absorbents. We found that the removal of triphosphate over Mg(2)FeCl-LDH mainly underwent the surface adsorption and the near-edge intercalation, with the practical removal amount (9-11 mg(P)/g) corresponding to 10-15% of the theoretical one. In contrast, Ca(2)FeCl-LDH removed a higher amount of triphosphate (56.4 mg(P)/g). The comprehensive analysis of the triphosphate-uptake products with XRD/XPS/FTIR reveals that Ca(2)FeCl-LDH dissolves first and then released Ca(2+) ions react with triphosphate (TPP) to form insoluble Ca-TPP precipitate. Combination of these two different removal mechanisms enables Mg(0.5)Ca(1.5)FeCl-LDH to take up 84.2mg(P)/g from aqueous solution under similar conditions.

Effective removal of selenate from aqueous solutions by the Friedel phase

This research has demonstrated that the Friedel phase, e.g. a chloride-containing hydrocalumite (Ca(2)Al(OH)(6)Cl(H(2)O)(2) x mH(2)O), can rapidly adsorb large amounts of SeO(4)(2-) (up to 1.37 mmol/g). SeO(4)(2-) is removed via anionic exchange, as evidenced by the expansion of the d-spacing from 0.78 nm of Cl-hydrocalumite to 0.97-0.98 nm of SeO(4)-hydrocalumite. The newly formed SeO(4)-adsorbed hydrocalumite is stable in water at pH 4-13, indicating the strong fixation of selenate within the phase. In contrast, intercalated selenate in the Freidel phase can be recovered by desorbing in the NaCl solution, which can also regenerate and recycle the used adsorbent. The findings in this research strongly suggest that the Friedel phase is a new, environmentally friendly and cost-effective adsorbent to adsorb selenate from wastewater streams and dilute solutions.

Stabilization of chromium-bearing electroplating sludge with MSWI fly ash-based Friedel matrices.

This work investigated the feasibility and effectiveness of MSWI fly ash-based Friedel matrices on stabilizing/solidifying industrial chromium-bearing electroplating sludge using MSWI fly ash as the main raw material with a small addition of active aluminum. The compressive strength, leaching behavior and chemical speciation of heavy metals and hydration phases of matrices were characterized by TCLP, XRD, FTIR and other experimental methods. The results revealed that MSWI fly ash-based Friedel matrices could effectively stabilize chromium-bearing electroplating sludge, the formed ettringite and Friedel phases played a significant role in the fixation of heavy metals in electroplating sludge. The co-disposal of chromium-bearing electroplating sludge and MSWI fly ash-based Friedel matrices with a small addition of active aluminum is promising to be an effective way of stabilizing chromium-bearing electroplating sludge.

CaFeAl mixed oxide derived heterogeneous catalysts for transesterification of soybean oil to biodiesel

CaAl layered double oxides (LDO) were prepared by co-precipitation and calcined at 750°C, and then applied to biodiesel production by transesterification reaction between methanol and soybean oil. Compared with characteristics of CaFe/LDO and CaAl/LDO, CaFeAl/LDO had the best performance based on prominent catalytic activity and stability, and achieved over 90% biodiesel yield, which stayed stable (over 85%) even after 8 cycles of reaction. The optimal catalytic reaction condition was 12:1M-ratio of methanol/oil, reaction temperatures of 60°C, 270rpm stirring rate, 60min reaction time, and 6% weight-ratio of catalyst/oil. In addition, the CaFeAl/LDO catalyst is insoluble in both methanol and methyl esters and can be easily separated for further reaction, turning it into an excellent alternative for biodiesel synthesis.

Improvement of ground granulated blast furnace slag on stabilization/solidification of simulated mercury-doped wastes in chemically bonded phosphate ceramics

This paper investigated the effectiveness of (ground granulated blast furnace slag) GGBFS-added chemically bonded phosphate ceramic (CBPC) matrix on the stabilization/solidification (S/S) of mercury chloride and simulated mercury-bearing light bulbs (SMLB). The results showed that the maximal compressive strength was achieved when 15% and 10% ground GGBFS was added for HgCl(2)-doped and SMLB-doped CBPC matrices, respectively. The S/S performances of GGBFS-added matrices were significantly better than non-additive matrices. As pore size was reduced, the leaching concentration of Hg(2+) from GGBFS-added CBPC matrix could be reduced from 697 microg/L to about 3 microg/L when treating HgCl(2). Meanwhile, the main hydrating product of GGBFS-added matrices was still MgKPO(4).6H(2)O. The improvement of S/S effectiveness was mainly due to physical filling of fine GGBFS particles and microencapsulation of chemical cementing gel.

Application and reactivation of magnetic nanoparticles in Microcystis aeruginosa harvesting.

This study developed a magnetic nanoparticles (MNPs) harvesting and reactivation technique for rapid cyanobacteria Microcystis aeruginosa separation. The harvesting of raw MNPs achieved high efficiency of 99.6% with the MNPs dosage of 0.58g MNPs/g dry-biomass, but gradually decreased to 59.1% when directly reused 5 times. With extra ultrasonic chloroform:methanol solvent treatment, the MNPs can be effectively reactivated for M. aeruginosa harvesting with 60% efficiency after 5 times reactivation and the separation efficiency kept above 93% with 0.20g MNPs/g dry-biomass dosage. The cyanobacteria-MNPs complex can be effectively disrupted by ultrasonic chloroform:methanol solvent treatment and the zeta potential was recovered for MNPs electrostatic attraction. The MNPs adsorption followed the Langmuir isotherm, and the maximum adsorption capacity and Langmuir constant was 3.74g dry-biomass/g and 311.64L/g respectively. This MNPs reactivation technique can achieve low energy separation and reduce MNPs consumption by 67%, providing potential engineering implementation for cyanobacterial biomass harvesting.

Enhanced remediation of Cr(VI)-contaminated soil by incorporating a calcined-hydrotalcite-based permeable reactive barrier with electrokinetics.

This paper describes the enhanced Cr(VI)-contaminated soil remediation via a combination of electrokinetics (EK) with a calcined-hydrotalcite-based permeable reactive barrier (PRB). First, this combination proved to be feasible, and remarkably facilitated Cr(VI) remediation in a column test. Then, lightly-to-severely (0.16-1.65 mg/g) Cr(VI)-contaminated soil was remediated in a simulated test with the calcined hydrotalcite as the PRB under an voltage of 10-30 V (i.e. an electric field intensity of 0.7-2.0 V/cm). The observations demonstrated that both PRB and EK are critical to efficient remediation and the high de-contamination efficiency is supposedly attributed to the synergistic effect, for which EK concentrates anionic chromate to the anode region and PRB media (calcined hydrotalcite) absorbs and immobilizes it. Thus we have shown that the combined PRB-EK system is highly adaptive and effective in remediation of a larger area contaminated with chromate and various anionic pollutants.

Efficient selective catalytic reduction of NO by novel carbon-doped metal catalysts made from electroplating sludge.

Electroplating sludges, once regarded as industrial wastes, are precious resources of various transition metals. This research has thus investigated the recycling of an electroplating sludge as a novel carbon-doped metal (Fe, Ni, Mg, Cu, and Zn) catalyst, which was different from a traditional carbon-supported metal catalyst, for effective NO selective catalytic reduction (SCR). This catalyst removed >99.7% NO at a temperature as low as 300 °C. It also removed NO steadily (>99%) with a maximum specific accumulative reduced amount (MSARA) of 3.4 mmol/g. Gas species analyses showed that NO removal was accompanied by evolving N2 and CO2. Moreover, in a wide temperature window, the sludge catalyst showed a higher CO2 selectivity (>99%) than an activated carbon-supported metal catalyst. Structure characterizations revealed that carbon-doped metal was transformed to metal oxide in the sludge catalyst after the catalytic test, with most carbon (2.33 wt %) being consumed. These observations suggest that NO removal over the sludge catalyst is a typical SCR where metals/metal oxides act as the catalytic center and carbon as the reducing reagent. Therefore, our report probably provides an opportunity for high value-added utilizations of heavy-metal wastes in mitigating atmospheric pollutions.