Xiaomin Dou

Effects of pyrolysis temperature on soybean stover- and peanut shell-derived biochar properties and TCE adsorption in water

Conversion of crop residues into biochars (BCs) via pyrolysis is beneficial to environment compared to their direct combustion in agricultural field. Biochars developed from soybean stover at 300 and 700 °C (S-BC300 and S-BC700, respectively) and peanut shells at 300 and 700 °C (P-BC300 and P-BC700, respectively) were used for the removal of trichloroethylene (TCE) from water. Batch adsorption experiments showed that the TCE adsorption was strongly dependent on the BCs properties. Linear relationships were obtained between sorption parameters (K(M) and S(M)) and molar elemental ratios as well as surface area of the BCs. The high adsorption capacity of BCs produced at 700 °C was attributed to their high aromaticity and low polarity. The efficacy of S-BC700 and P-BC700 for removing TCE from water was comparable to that of activated carbon (AC). Pyrolysis temperature influencing the BC properties was a critical factor to assess the removal efficiency of TCE from water.

Arsenate adsorption on three types of granular schwertmannite

Schwertmannite was synthesized on a 2 m(3)-scale and fabricated to irregular, cylindrical and spherical shape granules using drum granulation, extrusion and spray coating, respectively. The granules were systematically evaluated for As(V) removal from drinking water in terms of both performance and safety. The irregular and cylindrical shape granules (IS and CS) had larger schwertmannite loadings, higher porosity, more abundant pore structure and larger micropore volumes than those with a spherical shape (SS). As(V) adsorption kinetics on IS, CS and SS schwertmannite granules followed a pseudo-second order rate equation and two-stages of intraparticle diffusion. The rate parameters were in an order of IS > CS > SS granules. The faster uptake kinetics of the IS granules was due to their largest pore volume and interparticle porosity. Furthermore, adsorption capacities of 34, 21 and 5 mg/g, for IS, CS and SS granular schwertmannite samples were achieved at an initial As(V) concentration of 20 mg/L and adsorbent dose of 0.5 g/L. IS and CS samples performed much better over a wide pH range versus SS samples. Except for humic acid, PO4(3-) and SiO4(4-) did not inhibit As(V) adsorption on IS and CS granular specimens. SS samples worked poorly even in the absence or presence of co-existing anions. Regeneration was achieved using 0.1 M NaOH. The recycled IS and CS granular specimens can be used for 4 different cycles with no or nominal loss of adsorption capacity. Column experiments were also conducted. The IS, CS and SS granular specimens treated 8100, 4200 and 120 bed volumes (BVs) of contaminated water. No heavy metals leached from the packed granular adsorbent and appeared in the column effluent. Furthermore, the toxicity characteristic leaching procedure (TCLP) showed that the spent IS and CS granules were inert and could safely be disposed of in landfills. In short, irregular-shaped granules (IS) fabricated by drum granulation is a good candidate for arsenic removal from drinking water with a high future application potential.

Arsenate removal from water by zero-valent iron/activated carbon galvanic couples.

Galvanic couples composed of zero-valent iron and activated carbon (Fe(0)/AC) were investigated for As(V) removal from water. The effects of Fe(0) to AC mass ratio (FCR), solution pH, ionic strength and co-existing anions (phosphate, carbonate, silicate, nitrate, chloride and sulfate) and humic acid (HA) on As(V) removal were evaluated. The results showed that the optimum mass ratio was 1:1, and Fe(0)/AC with this ratio was more effective for As(V) removal than Fe(0) and AC alone at pH of 7 and ion strength of 0.03 M NaCl. The enhanced performance for As(V) removal was fulfilled through an accelerated corrosion process of Fe(0), which meant more corrosion products for efficient As(V) removal. The As(V) removal followed a pseudo-first order reaction. The rate constants (k) for 1:1 Fe(0)/AC and Fe(0) alone were 0.802 and 0.330 h(-1), respectively. Potentiodynamic polarization scans further confirmed that Fe(0) corrosion was promoted when Fe(0) was coupled with AC. Except silicates, other co-existing anions promoted As(V) removal. No reduction form of As (As(III) or As(0)) could be detected on iron corrosion products (ICPs) and in solutions. Identified ICPs included poorly crystallized lepidocrocite (gamma-FeOOH) and magnetite/maghemite (Fe(3)O(4)/gamma-Fe(2)O(3)) for both of Fe(0)/AC and Fe(0) systems. In conclusion, the Fe(0)/AC couple exhibited higher As removal performance than that of Fe(0) alone from water.

Surface complexation modeling and spectroscopic evidence of antimony adsorption on iron-oxide-rich red earth soils.

Few studies have investigated surface complexation of antimony (Sb) on natural sorbents. In addition, intrinsic acidic constants, speciation, and spectroscopic data are scarce for Sb sorption in soil. Only simple sorption models have been proposed to describe the sorption of Sb(V) on specific mineral surfaces. This study therefore assessed the mechanisms of Sb(III) and Sb(V) adsorption on natural red earth (NRE), a naturally occurring iron coated sand, at various pHs and Sb loadings. The Sb(V) adsorption followed typical anion adsorption curve with adsorption reaching maximum around pH 4-5, while no pH dependence was observed for Sb(III) sorption. The FT-IR spectra revealed that shifts in absorbance of the hydroxyl groups in iron-oxide were related to the Fe-O-Sb bonds and provided evidence for inner sphere bond formation. Direct evidence on the strong interaction of Sb(III) and Sb(V) with ≡Fe-O and ≡Al-O was observed from the decrease in Fe-2p, Al-2p, and Si-2p peaks of the X-ray photoelectron spectroscopy (XPS) data before and after Sb(V) and Sb(III) adsorption on NRE. Successful data modeling using the 2-pK diffuse double layer model (DDLM) with the FITEQL revealed that sorption occurs through the formation of bidentate mononuclear and binuclear complexes. Model simulations showed a high affinity to the ≡FeOH sites at high Sb loadings, whereas at low loadings, both≡ FeOH and ≡AlOH sites showed similar affinities to Sb. In the case of Sb(V), multilayer formation was also revealed in addition to surface complexation by the isotherm data fitted with the Freundlich model and two sites Langmuir equations, which indicated heterogeneous multilayer adsorption of Sb(V) on NRE.

Removal of arsenate from water by using an Fe-Ce oxide adsorbent: effects of coexistent fluoride and phosphate.

The Langmuir two-site equation, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure spectroscopy have been employed to study the competitive behaviors of fluoride (F) and phosphate (P) in relation to arsenate adsorption on an Fe-Ce adsorbent as well as the mechanisms involved. The two-site isotherm revealed the presence of two kinds of adsorption sites with different binding affinities for arsenate. Both the total and low-binding-energy maximum adsorption capacities (Q and Q(1)) of arsenate decreased significantly even at a molar ratio of As/P=1:0.1. The coexistence of F, only influenced the total Q of arsenate at high simultaneous F concentrations. The fact that Fe-Ce released 0.15-0.24 mmol sulfate for every mmol arsenate adsorbed suggested that, while sulfate groups might have played a role for adsorption, surface hydroxyl groups should be the major active sites. The XPS results indicated that arsenate and P are mainly adsorbed through the substitution of Fe surface active sites, while F is mainly adsorbed through substitution of Ce surface active sites. The As k-edge EXAFS data show that the second peak of Fe-Ce after arsenate adsorption is As-Fe shell, which further supported that arsenate adsorption occurs mainly at the Fe surface active sites.

Mechanisms of antimony adsorption onto soybean stover-derived biochar in aqueous solutions

Limited mechanistic knowledge is available on the interaction of biochar with trace elements (Sb and As) that exist predominantly as oxoanions. Soybean stover biochars were produced at 300 °C (SBC300) and 700 °C (SBC700), and characterized by BET, Boehm titration, FT-IR, NMR and Raman spectroscopy. Bound protons were quantified by potentiometric titration, and two acidic sites were used to model biochar by the surface complexation modeling based on Boehm titration and NMR observations. The zero point of charge was observed at pH 7.20 and 7.75 for SBC300 and SBC700, respectively. Neither antimonate (Sb(V)) nor antimonite (Sb(III)) showed ionic strength dependency (0.1, 0.01 and 0.001 M NaNO3), indicating inner sphere complexation. Greater adsorption of Sb(III) and Sb(V) was observed for SBC300 having higher -OH content than SBC700. Sb(III) removal (85%) was greater than Sb(V) removal (68%). Maximum adsorption density for Sb(III) was calculated as 1.88 × 10(-6) mol m(-2). The Triple Layer Model (TLM) successfully described surface complexation of Sb onto soybean stover-derived biochar at pH 4-9, and suggested the formation of monodentate mononuclear and binuclear complexes. Spectroscopic investigations by Raman, FT-IR and XPS further confirmed strong chemisorptive binding of Sb to biochar surfaces.

A novel application of H2O2–Fe(II) process for arsenate removal from synthetic acid mine drainage (AMD) water

Batch experiments were carried out to investigate the influences of H(2)O(2)/Fe(II) molar ratio, pH, sequence of pH adjustment, initial As(V) concentration, and interfering ions on As(V) removal in H(2)O(2)-Fe(II) process from synthetic acid mine drainage (AMD). The optimum H(2)O(2)/Fe(II) molar ratio was one for arsenate removal over the pH range of 4-7. Arsenate removal at pH 3 was poor even at high Fe(II) dosage due to the high solubility of Fe(III) formed in situ. With the increase of Fe(II) dosage, arsenate removal increased progressively before a plateau was reached at pH 5 as arsenate concentration varied from 0.05 to 2.0mgL(-1). However, arsenate removal was negligible at Fe/As molar ratio <3 and then experienced a striking increase before a plateau was reached at pH 7 and arsenate concentration ≥1.0mgL(-1). The co-occurring ions exerted no significant effect on arsenate removal at pH 5. The experimental results with synthetic AMD revealed that this method is highly selective for arsenate removal and the co-occurring ions either improved arsenate removal or slightly depressed arsenate removal at pH 5-7. The extended X-ray absorption fine structure (EXAFS) derived As-Fe length, 3.27-3.30Å, indicated that arsenate was removed by forming bidentate-binuclear complexes with FeO(OH) octahydra. The economic analysis revealed that the cost of the H(2)O(2)-Fe(II) process was only 17-32% of that of conventional Fe(III) coagulation process to achieve arsenate concentration below 10μgL(-1) in treated solution. The results suggested that the H(2)O(2)-Fe(II) process is an efficient, economical, selective and practical method for arsenate removal from AMD.

Natural and synthesised iron-rich amendments for As and Pb immobilisation in agricultural soil

The immobilisation of heavy metals in contaminated soils is a promising alternative to conventional remediation techniques. Very few studies have focused on the use of iron-rich nanomaterials and natural materials for the adsorption of toxic metals in soils. Synthesised iron-rich nanomaterials (Fe and Zr–Fe oxides) and natural iron-rich materials (natural red earth; NRE) were used to immobilise As and Pb in contaminated agricultural soil. Total concentrations of As and Pb in the initial soil (as control) were 170.76 and 1945.11 mg kg−1, respectively. Amendments were applied into the soil at 1, 2.5 and 5% (w/w) in triplicate and incubated for 150 days. Except for the NRE-amended soil, soil pH decreased from 5.6 to 4.9 with increasing application rates of Fe and Zr–Fe oxides. With addition of Fe and Zr–Fe oxides at 5%, the ammonium acetate (NHO4Ac)-extractable Pb was greatly decreased by 83 and 65% compared with NRE addition (43%). All subjected amendments also led to a decrease in NHO4Ac-extractable As in the soils, indicating the high capacity of As immobilisation. Soil amended with NRE showed a lower ratio of cy19:0 to 18:1ω7c, indicating decreased microbial stress. The toxicity characteristic leaching procedure produced results similar to the NHO4Ac extraction for As and Pb. The NRE addition is recommended for immobilising heavy metals and maintaining biological soil properties.

Performance and mass transfer of aqueous fluoride removal by a magnetic alumina aerogel

Magnetic alumina aerogel (MAA) was successfully synthesized and evaluated for F− removal from water. The adsorbent was characterized by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), vibration sample magnetometry (VSM), X-ray photoelectron spectroscopy (XPS), FTIR, and Raman spectroscopy. The magnetic alumina aerogel sample has a BET surface area of 215.1 m2 g−1 and meso- to macro-sized pores ranging from 50 nm to 200 nm. A saturation magnetization of 19.8 emu g−1 was observed, which makes the separation of the adsorbent realizable after batch adsorption. Boehmite and magnetite phases were identified in the adsorbent. A core–shell structure is favored and additional studies are suggested. Aggregated aerogel nanoparticles composed of boehmite are the inner portion. Magnetite nanoparticles discretely and partly cover the alumina by leaving portions of aluminas surface exposed to the ambient environment. This adsorbent has a moderately high adsorption capacity of 32.1 mg F− per g adsorbent at pH 5.0. Batch studies revealed that fluoride adsorption followed the pseudo-second-order kinetics model. The intraparticle mass diffusion was fitted well using a homogeneous surface diffusion model (HSDM), and the intraparticle surface diffusion coefficients (Ds) were numerically determined. The evolution of dimensionless radial F− concentration profiles inside particles was also simulated. General co-existing anions did not inhibit F− uptake by MAA except for a slight inhibition by HCO3− and PO43−. Spiking experiments demonstrated that MAA effectively removed F− when treating simulated well water contained 1.83 mg L−1 F−. Al 2p and Fe 2p XPS spectra of MAA before and after F− removal demonstrated that both the alumina and iron oxide phases contributed to F− surface adsorption. The adsorption performance and easy separation of MAA show this adsorbent is a promising candidate for fluoride removal from water.

Fabrication of spherical biochar by a two-step thermal process from waste potato peel

The aim of this study was to develop a new approach for the preparation of spherical biochar (SBC) by employing a two-step thermal technology to potato peel waste (PPW). Potato starch (PS), as a carbon-rich material with microscale spherical shape, was separated from PPW as a precursor to synthesizing SBC. The synthesis process comprised (1) pre-oxidization (preheating under air) of PS at 220 °C and (2) subsequent pyrolysis of the pretreated sample at 700 °C. Results showed that the produced SBC successfully retained the original PS morphology and that pre-oxidization was the key for its shape maintenance, as it reduced surface tension and enhanced structural stability. The SBC possessed excellent chemical inertness (high aromaticity) and uniform particle size (10-30 μm). Zero-cost waste material with a facile and easy-to-control process allows the method to be readily scalable for industrialization, while offering a new perspective on the full use of PPW.