Nan Xu

Comparison of the characteristics and mechanisms of Hg(II) sorption by biochars and activated carbon.

Two biochars were produced from bagasse and hickory chips (referred to as BB and HCB, respectively) and evaluated for their sorption ability of Hg(II) in aqueous solution. A commercial activated carbon (AC) which is commonly used for Hg(II) removal was included for comparison. Both biochars showed higher sorption capacities than AC, following the trend of BB>HCB>AC. The sorption of Hg(II) by BB and AC was mainly attributed to the formation of (COO)2Hg(II) and (O)2Hg(II). As a result, the adsorption capacity of Hg(II) by BB decreased 17.6% and 37.6% after COOH and OH were blocked, respectively and that of Hg(II) by AC decreased 6.63% and 62.2% for COOH and OH hindered, respectively. However, blocking the function groups had little effect on the Hg removal by HCB since sorption of Hg(II) by HCB was mainly resulted from the π electrons of CC and CO induced Hg-π binding. Further X-ray photoelectron spectroscopy analysis indicated the possibility of reduction of the Hg(II) to Hg(I) by phenol groups or π electrons during the removal of Hg(II) by both biochars. In conclusion, biochar is more effective than activated carbon in removing Hg(II) and there exists a high potential that biochar can be a substitute of activated carbon for removal of Hg(II) from wastewater.

A Review of Molybdenum Adsorption in Soils/Bed Sediments: Speciation, Mechanism, and Model Applications

Mo is an essential trace element for both plants and animals in low concentrations (<5 ppm). However, provoked by uncontrolled industrial waste releases in freshwater or seawater, it is plausible that excessive availability of soluble Mo(VI) would be potentially toxic. In the environment, soluble Mo(VI) is mainly present in anionic forms of molybdate (MoO4 2−) and/or tetrathiomolybdate (MoS4 2−). The fate and transport of soluble Mo(VI) anions in surface and subsurface aquatic environments is typically controlled by adsorption in acidic soils and sediment. As such, the ability of soils/bed sediments to retain Mo(VI) is a key to determine its general mobility in the aquatic environment. This article reviews the sources and distribution of Mo speciation in solution and Mo(VI) anions adsorption mechanisms in soils and bed sediments, and evaluates the surface adsorption complexation models at the solid-water interface to estimate Mo(VI) anions adsorption in these chemical systems. Mo(VI) anions adsorption mechanisms included MoO4 2− and MoS4 2− adsorption by several prevailing adsorbent contents (including clay, Fe, Al oxides, iron sulfide, manganese oxides, and organic matter) of soils and bed sediments, and the influence of the competitive adsorption of other anions (e.g., sulfate, selenate, phosphate, arsenate, silicate, or tungstate). Models to estimate Mo(VI) anions adsorption include the triple layer model (TLM), the diffuse layer model (DLM), the constant capacitance surface complexation model (CCM), and charge distribution multisite complexation model (CD-MUSIC).

Facilitated transport of anatase titanium dioxides nanoparticles in the presence of phosphate in saturated sands.

Soil and water environments are inevitably contaminated by the excess of artificial nanoparticles (NPs) and phosphorus (P) fertilizers. There is a possibility of phosphate facilitating or inhibiting the transport of nanoparticles titanium dioxides (nTiO2). It is a great urgency and high priority to investigate the nTiO2 retention mechanisms and accurately describe the transport of nTiO2 in the presence of phosphate. Anatase nTiO2 with two sizes of 20 and 50nm through the saturated porous sand columns were observed under the conditions (0-50mM NaNO3 electrolyte, influent P concentrations of 0.10mM and 2.0mM, pH 6.5 and 7.5). The experimental results show the phosphate favor the dispersion of nTiO2, and consequently improve their transport patterns. The likely mechanism is that phosphate adsorption increasing the negative charge on the surface promotes the transportability of nTiO2 resulting from the low deposition rate and attachment efficiency of NPs. In particular, the facilitated transport of nTiO2 (50nm) is greater than those relative smaller as 20nm. In addition, this enhancement of nTiO2 transportability by phosphate at pH 6.5 is increased at higher pH of 7.5 due to the more negative zeta potential of surface, which indicates the potential risks to groundwater systems.

Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions☆

Land application of biochar has been increasingly recommended as a powerful strategy for carbon sequestration and soil remediation. However, the biochar particles, especially those in the nanoscale range, may migrate or carry the inherent contaminants along the soil profile, posing a potential risk to the groundwater. This study investigated the transport and retention of wood chip-derived biochar nanoparticles (NPs) in water-saturated columns packed with a paddy soil. The environmentally-relevant soil solution chemistry including ionic strength (0.10-50xa0mM), electrolyte type (NaCl and CaCl2), and natural organic matter (0-10xa0mgxa0L-1 humic acid) were tested to elucidate their effects on the biochar NPs transport. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl2 electrolyte being more effective than NaCl in decreasing biochar NPs transport. The retained biochar NPs in NaCl was re-entrained (∼57.7%) upon lowering transient pore-water ionic strength, indicating that biochar NPs were reversibly retained in the secondary minimum. In contrast, negligible re-entrainment of biochar NPs occurred in CaCl2 due to the primary minimum and/or particle aggregation. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. The transport behaviors of biochar NPs can be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site, and irreversible retention for the other site. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the importance of understanding the mobility of biochar NPs in natural soils for accurately assessing their environmental impacts.

Mechanisms of phosphate retention by calcite: effects of magnesium and pH

PurposeSorption and precipitation of phosphate are important processes in controlling fate of phosphorus (P) in P-fertilized soils, especially those affected by magnesium (Mg) ions.Materials and methodsThe interaction between Mg(II) (0.42 and 8.33xa0mM) ions and phosphate (0.32 and 6.45xa0mM) at the calcite–water interface were investigated with various pH values from 6.0 to 12.0, using a combination of sorption envelopes, Fourier transform infrared spectroscopy, and X-ray diffraction.Results and discussionAmorphous calcium phosphate, dibasic calcium phosphate dihydrate, and hydroxyapatite are formed at high phosphate concentration (6.45xa0mM) and high pH (>8.0). The presence of low Mg(II) ion level (0.42xa0mM) had little effect on phosphate sorption. When Mg(II) ions increased to 8.33xa0mM, phosphate retention was inhibited in the weak acid condition since incorporation of Mg(II) ions kinetically hinders precipitation resulting in greater solubility of calcium phosphate while high pH favors Mg adsorption to provide more =Mg sites and OH functional groups on the surface of calcite, which enhanced the formation of Mg–P phases. The likely mechanism is attributed to the different surface terminations of calcite sorbed by phosphate at pHu2009<u20098.0 and pHu2009>u20098.0 in the presence of Mg(II) ions.ConclusionsOur experimental results suggested that soil pH, initial concentration of phosphate, and the presence of Mg(II) ions and calcite play an important role to affect the fate of phosphate in P-fertilized soils.

Transport and aggregation of rutile titanium dioxide nanoparticles in saturated porous media in the presence of ammonium

The widely used artificial nanoparticles (NPs) and the excess of ammonium (NH4+) fertilizers are easily released into the natural environment. So, clarifying the mobility of NPs in the presence of NH4+ is therefore of great urgency and high priority. Currently, few studies focus on the transport and deposition of nanoparticle titanium dioxide (nTiO2) in single and binary systems containing NH4+, especially describing this process by a mathematical model. In this work, the comparison between the transport and retention of rutile nTiO2 in single and binary electrolyte solutions of NH4Cl and/or NaCl (0.5-50xa0mM) were conducted at pH 6.0 and 8.0 through running the column experiments. Experimental results show that the aggregation and retention of nTiO2 in solution containing mono-valence cations obeys the order as follows: NH4+xa0>xa0Na+xa0>xa0Na+xa0+xa0NH4+ at the same ion strength (IS). It is attributed to the lower critical coagulation concentration (CCC) of rutile nTiO2 in NH4+ than that in Na+ solution. In particular, the simultaneous presence of NH4+ and Na+ favors the transportability of nTiO2 due to the strong competitive adsorption on the surface of NPs. The two-site kinetic retention model provides the good simulation for their transport behavior. The likely mechanism is that the secondary energy minimum of nTiO2 in NH4+ system associated with the greater K2 at surface Site 2 (from model) on sand can be explained for the more reversible deposition. Ammonium leachate associated with NPs can thus be considered a serious concern.

Protein Staining Agents from Cationic and Neutral Luminescent Iridium(III) Complexes.

Seven luminescent iridium(III) complexes were prepared to investigate the relationships between chemical structures and properties of protein staining. For the first time, the effect of the main ligand, the π conjugation effect of the ancillary ligand, and the charge effect of organometallic complexes on protein staining has been revealed. Most importantly, this study gives the first experimental evidence of the potential applications of charge-neutral organometallic complexes in protein staining, which could open an avenue of exploiting novel protein staining agents in the future.

Retention of phosphorus on calcite and dolomite: speciation and modeling

The intensive application of phosphate fertilizers in agriculture has created an important source of diffuse phosphorus pollution. The interaction of phosphorus with carbonate minerals plays a role in the fate and transport of phosphorus in soil. The object of the present study was to investigate the speciation of phosphorus on two common carbonate minerals, calcite and dolomite, using a combination of batch experiments, ATR-FTIR spectroscopy, XANES analysis, and diffuse layer modeling. Within the pH range 6.0–7.0, the retention of phosphorus by calcite and dolomite is mainly attributed to the formation of amorphous calcium phosphate (Ca3(PO4)2, ACP), dibasic calcium phosphate (CaHPO4·2H2O, DCP), and hydroxyapatite (Ca5(PO4)3OH, HAP). At pH ≥ 8.0 the immobilized phosphorus takes the form of complexes =CaPO4Ca0/=sCaPO4Ca0 on the surface of calcite, followed by the formation of Ca–P phases, including ACP, DCP, and HAP, with increasing phosphorus levels (>2 mg L−1). However, the dolomite surface is initially dominated by the adsorption complex =MgHPO4Ca+ at =Mg sites, and at higher phosphorus levels it then grows due to Ca–P phases and the formation of newberyite (MgHPO4). It is interesting to note that the Mg content in dolomite favors the rapid growth of DCP at phosphorus levels >200 mg L−1. As a result, at pH ≥ 8.0, dolomite shows a stronger capacity for immobilizing phosphorus than does calcite. Dolomite therefore serves as a better phosphorus sink than calcite in calcareous soil environments.

Fabrication of mesoporous Al-SBA-15 as a methylene blue capturer via a spontaneous infiltration route

Different amounts of aluminum (Al) species were infiltrated and incorporated into SBA-15 by grinding the mixture of aluminum nitrate and SBA-15 followed with a subsequent calcination. A novel series of methylene blue adsorbents were thus obtained and characterized by X-ray diffraction, N2 adsorption–desorption, Fourier transform infrared and 27Al Nuclear Magnetic Resonance (NMR) techniques. Results show that the newly synthesized Al-SBA-15 materials have well-preserved mesostructures, high BET surface areas, enlarged pore sizes and predominantly tetrahedrally coordinated Al species. All Al-SBA-15 materials show a greater adsorption capacity to methylene blue than SBA-15, where the sample (Al/Si = 0.05) has the largest capacity to remove MB from water solution. Diffuse reflectance measurements, Langmuir and Freundlich equations and the pseudo-second-order kinetic model were further used to describe the adsorption behavior of MB onto SBA-15 and Al-SBA-15 materials. Finally, recycling tests were performed to evaluate the stability of the Al-SBA-15 material and the textural and NMR properties of the used samples were also detected.

Mechanisms of Phosphate Removal by Synthesized Calcite

Calcite was synthesized through different drying processes, and characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). By bath experiments, the mechanisms of phosphate removal by synthesizing calcite were investigated. It showed that synthesis calcite had a strong capability to remove phosphate from solution. The adsorption of phosphate mainly depended on the total concentration of phosphate and pH in solution. Magnesium tended to the phosphate adsorption during pH range of 8-13.5; while it inhibited the adsorption at around pH 7. The experimental results suggested that the synthesized calcite with aged process could effectively remove the phosphate from an alkaline aqueous solution.