Baoyu Gao

Adsorption of hexavalent chromium from aqueous solution by modified corn stalk: a fixed-bed column study.

Continuous fixed-bed column studies were carried out by using modified corn stalk (MCS) as an adsorbent for the removal of Cr(VI) from aqueous solution. The effect of various parameters like bed depths (1.4, 2.2 and 2.9 cm), flow rate (5, 10 and 15 mL/min), influent Cr(VI) concentrations (100, 200 and 300 mg/L) and influent solution pH (2.66, 4.91 and 5.66) was investigated. The exhaustion time increased with increase of bed depth, decrease of flow rate and influent concentration. The Adams-Bohart, Thomas and Yoon-Nelson models were applied to the adsorption under varying experimental conditions to predict the breakthrough curves and to evaluate the model parameters of the fixed-bed column that are useful for process design. The Thomas and Yoon-Nelson models were in good agreement with the experimental data. The MCS column study states the value of the excellent adsorption capacity for the removal of Cr(VI) from aqueous solution.

Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide

A series of sols consisting of aluminum magnesium mixed hydroxide (AMH) nanoparticles with various Mg/Al molar ratios were prepared by coprecipitation. The use of AMH as adsorbent to remove Cr(VI) from aqueous solution was investigated. Adsorption experiments were carried out as a function of the Mg/Al molar ratio, pH, contact time, concentration of Cr(VI) and temperature. It was found that AMH with Mg/Al molar ratio 3 has the largest adsorption efficiency due to the smallest average particle diameter and the highest zeta potential; AMH was particularly effective for the Cr(VI) removal in a pH range from acid to slightly alkaline, even though the most effective pH range was between 2.5 and 5.0. The adsorption of Cr(VI) on AMH reached equilibrium within 150 min. The saturated adsorption capacities of AMH for Cr(VI) were 105.3-112.0mg/g at 20-40 degrees C. The interaction between the surface sites of AMH and the Cr(VI) ions may be a combination of both anion exchange and surface complexation. The pseudo-second-order model best described the adsorption kinetics of Cr(VI) onto AMH. The results showed that AMH can be used as a new adsorbent for Cr(VI) removal which has higher adsorption capacity and faster adsorption rate at pH values close to that at which pollutants are usually found in the environment.

Removal of fluoride by hydrous manganese oxide-coated alumina: performance and mechanism.

A novel hydrous-manganese-oxide-coated alumina (HMOCA) material was prepared through a redox process. The adsorbent was characterized by SEM, BET surface area measurement, XRD, pH(PZC) measurement, FTIR spectroscopy, and XPS. The manganese oxides were amorphous and manganese existed mainly in the +IV oxidation state. Batch and column experiments were carried out to investigate the adsorption potential of the adsorbent. Fluoride adsorption onto HMOCA followed the pseudo-second-order equation well with a correlation coefficient greater than 0.99. Both external and intraparticle diffusion contributed to the rate of transfer and removal. The adsorption of fluoride was thought to take place mainly by ion-exchange. Optimum removal of fluoride occurred in a pH range of 4.0-6.0. The maximum adsorption capacity calculated from the Langmuir model was 7.09 mg/g. The presence of HCO(3)(-), SO(4)(2-) and PO(4)(3-) had negative effects on the adsorption of fluoride. The adsorbed fluoride can be released by alkali solution. Column studies were performed and 669 bed volumes were treated with the effluent fluoride under 1.0mg/L at an influent F(-) concentration of 5.0mg/L and flow rate of 2.39 m(3)/(m(2)h) (empty bed contact time=7.5 min).

Comparison of coagulation behavior and floc structure characteristic of different polyferric-cationic polymer dual-coagulants in humic acid solution

Three polyferric-cationic polymer dual-coagulants were comparatively evaluated in terms of coagulation behavior and floc structure characteristic in the coagulation of humic acid (HA) solution. The first dual-coagulant, PFC-PDADMAC, was prepared by premixing of polyferric chloride (PFC) and polydiallyldimethylammonium (PDADMAC) before dosing. The other two were achieved by dosing PFC and PDADMAC in different order. For the given neutral condition, all three dual-coagulants gave similar HA removal before reaching optimal dosage. The strongest charge neutralization and narrowest effective coagulation dosage range were obtained when PFC was dosed firstly. While the weakest charge neutralization and the broadest effective coagulation dosage were obtained when PDADMAC was used as the primary coagulant. The HA removal of all three dual-coagulants was slightly pH dependent for optimum coagulant doses. Fe(III) hydrolysis species distributions of the dual-coagulants in coagulation process were measured by ferron method. PFC-PDADMAC gave the highest content of active Fe(III) coagulating species which is responsible for the coagulation performance of ferric coagulant. The evolution of floc size and floc fractal dimension (Df) in coagulation process was measured under optimum dose and neutral condition by laser diffraction instrument and small-angle laser light scattering (SALLS), respectively. All three dual-coagulants gave similar final floc size but different floc growth rate and floc structure. Both the growth rate and Df were in the same order: PFC dosed firstly>PDADMAC dosed firstly>PFC-PDADMAC.

Adsorption kinetics and isotherm of anionic dyes onto organo-bentonite from single and multisolute systems.

The performances of polydiallydimethylammonium modified bentonite (PDADMA-bentonite) as an adsorbent to remove anionic dyes, namely Acid Scarlet GR (AS-GR), Acid Turquoise Blue 2G (ATB-2G) and Indigo Carmine (IC), were investigated in single, binary and ternary dye systems. In adsorption from single dye solutions with initial concentration of 100 micromol/L, the dosage of PDADMA-bentonite needed to remove 95% dye was 0.42, 0.68 and 0.75 g/L for AS-GR, ATB-2G and IC, respectively. The adsorption isotherms of the three dyes obeyed the Langmuir isotherm model with the equilibrium constants of 0.372, 0.629 and 4.31 L/micromol, the saturation adsorption amount of 176.3, 149.2 and 228.7 micromol/g for ATB-2G, IC and AS-GR, respectively. In adsorption from mixed dye solutions, the isotherm of each individual dye followed an expanded Langmuir isotherm model and the relationship between the total amount of dyes adsorbed and the total equilibrium dye concentration was interpreted well by Langmuir isotherm model. In the region of insufficient dosage of PDADMA-bentonite, the dye with a larger affinity was preferentially removed by adsorption. Desorption was observed in the kinetic curve of the dye with lower affinity on PDADMA-bentonite surface by the competitive adsorption. The kinetics in single dye solution and the total adsorption of dyes in binary and ternary dye systems nicely followed pseudo-second-order kinetic model.

Equilibrium and kinetic adsorption study of the adsorptive removal of Cr(VI) using modified wheat residue

A new adsorbent modified from wheat residue was synthesized after reaction with epichlorohydrin and triethylamine by using the modifying agents of diethylenetriamine in the presence of organic medium of N,N-dimethylformamide. The performance of the modified wheat straw (MWS) was characterized by Fourier transform infrared spectroscopy and point of zero charge analysis. The adsorption was investigated in a batch adsorption system, including both equilibrium adsorption isotherms and kinetics. Results showed that MWR had great anion-adsorbing capacity, due to the existence of a large number of introduced amino groups, and the value of pH(PZC) was around 5.0. Equilibrium data were analyzed using the Langmuir, Freundlich, and Temkin isotherm models and were found to be best represented by the Freundlich isotherm model. Evaluation of the adsorption process identified its endothermic nature. The maximum adsorption capacity of MWS for the removal of Cr(VI) was 322.58mg/g at 328K, indicating that MWS has high chromium removal efficiency, compared to other adsorbents reported. The kinetics of adsorption followed the pseudo-second-order kinetic equation. The mechanism of adsorption was investigated using the intraparticle diffusion model. Thermodynamic parameters (free energy change, enthalpy change, and entropy change) revealed that the adsorption of Cr(VI) onto MWS was endothermic and spontaneous; additionally, the adsorption can be characterized as an ion-exchange process. The results suggest that MWS is an inexpensive and efficient adsorbent for removing Cr(VI) ions from aqueous solution.

Adsorption kinetics and desorption of Cu(II) and Zn(II) from aqueous solution onto humic acid

The adsorption kinetics of Cu(II) and Zn(II) from aqueous solution onto humic acid (HA) were investigated under different conditions. The results showed that HA was particularly effective for adsorbing heavy metals and that the effective pH range was above 6. The adsorption force was not single and dependent on solution pH. The experimental data were found to comply with the pseudo-second-order kinetic model with adsorption capacity, q(e cal) more close to the measured results. Increment of initial adsorbate concentration (1-4 mg/L) and temperature (283-303 K) are both favorable to adsorption process. Based on the desorption experiments, it is further indicated that the main adsorption force was complexation effect between organic ligands of HA surface and metal ions and the percentage of desorption is 50% for Cu and 30% for Zn, respectively.

Characterization of floc size, strength and structure in various aluminum coagulants treatment.

Flocs generated in various aluminum coagulants treatment exhibit difference in size, strength and structure. The formation, breakage and re-formation of flocs were investigated by using a laser diffraction instrument (Malvern Mastersizer 2000, Malvern, UK). Different Al coagulants--conventional Al salt (AlCl(3)), polyaluminum chloride (PAC-1) and the purified polyaluminum chloride (PAC-2)--were investigated for the coagulation of humic acid (HA). The results showed the properties of HA flocs depend on which species of aluminum used. The monomeric and dimeric aluminum species (denoted as Al(mono)) formed the largest and strongest HA flocs with longest growth time. The HA flocs formed by Al(13)O(4)(OH)(24)(7+) (Al(13) for short) is the smallest and weakest. The properties of HA flocs made it possible to investigate the relationship between the flocs growth process and aluminum species.

Evaluation of aluminum-silicate polymer composite as a coagulant for water treatment

Aluminum-silicate polymer composite (PASiC), a new kind of inorganic coagulant, was produced by two approaches: (1) hydroxylation of the mixture of AlCl3 and fresh polysilicate (PASiCc); (2) hydroxylated polyaluminum-chloride (PAC) combined with fresh polysilicate (PASiCm). The PASiC products had the following properties: Al2O3 content = 6.40-7.30%, SiO2 content = 0.40-0.82%, Al/Si ratios = 10-20, basicity (OH/Al molar ratio, denoted B) = 1.2-2.0. The coagulation behaviour of PASiC and PAC under conditions typical for coagulation and flocculation in water treatment were investigated by studying the rate of floc size development, the variety of streaming current value, the efficiency of turbidity removal and the effect of pH on the turbidity removal efficiency, and the stability of PASiC. The results detailed in this study suggest that, compared with PAC, PASiC may enhance aggregating efficiency and give better coagulating effects, but weaken charge effectiveness in coagulation process or become unstable when stored for longer time, especially at higher B value and lower Al/Si ratio. The coagulating effect of PASiC is closely linked to the preparation procedure. With the increase of B value and the decrease of Al/Si ratio, the coagulation efficiency of PASiC increases, and at the same B value and Al/Si ratio, PASiCc seems to give a little better coagulation effect than PASiCm but less stability. The Al/Si ratio should not be too low or the B value should not be too high, otherwise, the PASiC products tend to become cloudy or partly gelatinous, which will make them loss some coagulation efficiency.

Impact of COD/N ratio on nitrous oxide emission from microcosm wetlands and their performance in removing nitrogen from wastewater

Constructed wetlands (CWs) are considered to be important sources of nitrous oxide (N(2)O). In order to investigate the effect of influent COD/N ratio on N(2)O emission and control excess emission from nitrogen removal, free water surface microcosm wetlands were used and fed with different influent. In addition, the transformation of nitrogen was examined for better understanding of the mechanism of N(2)O production under different operating COD/N ratios. It was found that N(2)O emission and the performance of microcosm wetlands were significantly affected by COD/N ratio of wastewater influent. Strong relationships exist between N(2)O production rate and nitrite (r=0.421, p<0.01). During denitrification process, DO concentration crucially influences N(2)O production rate. An optimal influent COD/N ratio was obtained by adjusting external carbon sources for most effective N(2)O emission control and best performance of the CWs in nitrogen removal from wastewater. It is concluded that under the operating condition of COD/N ratio=5, total N(2)O emission is minimum and the microcosm wetland is most effective in wastewater nitrogen removal.