Brian A. Dempsey

A Review of the Use of Chitosan for the Removal of Particulate and Dissolved Contaminants

Abstract Chitosan has unique properties among biopolymers, especially due to the presence of primary amino groups. Chitosan has been used for the chelation of metal ions in near‐neutral solution, the complexation of anions in acidic solution (cationic properties due to amine protonation), the coagulation of negatively charged contaminants under acidic conditions, and for precipitative flocculation at pH above the pKa of chitosan. The coagulation and flocculation properties can be used to treat particulate suspensions (organic or inorganic) and also to treat dissolved organic materials (including dyes and humic acid). This paper will give an overview of the principal results obtained in the treatment of various suspensions and solutions: (a) bentonite suspensions; (b) organic suspensions; (c) anionic dye solutions; and (d) humic acid solutions. Stoichiometry and charge restabilization were determined for the coagulation of humic acid, kaolin, and organic dyes with chitosan, indicating charge neutralization as the dominant mechanism for removal. Charge patch destabilization and bridging mechanisms were inferred in other cases, based on the effects of the apparent molecular weight of the chitosan preparations and effectiveness of sub‐stoichiometric doses of chitosan. For dye solutions, results showed that color can be removed either by sorption onto solid‐state chitosan or by coagulation‐flocculation using dissolved‐state chitosan; the reactivity of amine groups was significantly increased when dissolved chitosan was used. For humic materials, chitosan can be used as a primary coagulant or as a flocculant after coagulation with alum or other inexpensive coagulants. The influence of the degree of deacetylation and the molecular weight of chitosan on its performance as coagulant/flocculant is illustrated by several examples.

Reactions of ferrous iron with hematite

Abstract The adsorption of Fe(II) onto hematite was measured as a function of pH, surface area, and time. The effects of anions (chloride, sulfate, or nitrate) and of Zn(II) were also determined. All experiments were conducted under strict anoxic conditions with 5 or 30 days for equilibration. Results showed that immobilization of Fe(II) on hematite consists of a fast sorption process and one or more slow processes, which probably include both sorption and formation of new phases. Sorption occurred at pH values as low as 4, which has not been reported in existing literature. Some Fe(II) could not be extracted after 20 h with 0.5 N HCl. In the presence of 0.01 M NaCl, all of the added Fe(II) was recovered when pH was below 6, but either 100% or less than 25% of added Fe(II) was recovered when pH was greater than 6. These results are consistent with auto-catalytic formation of magnetite, which was stable relative to hematite for pH above 5.9. However, when sulfate was greater than 1 mM, unextracted Fe(II) was observed at pH above 5 where only approximately 15% of added Fe(II) was recovered by a 0.5 N HCl extraction; these results could not be explained by precipitation of magnetite nor of known sulfate phases. Based on these results, existing models for adsorption of Fe(II) onto ferric oxides (based on experiments of several hours to a day) are not accurate for prediction of environmentally significant Fe(II) reactions with ferric oxides, when much longer times are available for reaction. There was no competition between Zn(II) and Fe(II) for 0.25 mM or less and 90 m 2 l −1 hematite. Zn(II) was completely recovered using 0.5 N HCl for every condition that was tested.

Sorption kinetics of Fe(II), Zn(II), Co(II), Ni(II), Cd(II), and Fe(II)/Me(II) onto hematite.

The reactions of Fe(II) and other divalent metal ions including Zn, Co, Ni, and Cd on hematite were studied in single and competitive binary systems with high sorbate/sorbent ratios in 10 mM PIPES (pH 6.8) solution under strict anoxic conditions. Adsorbed Me(II) was defined as extractable by 0.5 N HCl within 20 h, and fixed Me(II) was defined as the additional amount that was extracted by 3.0 N HCl within 7 days. Binary systems contained Fe(II) plus a second metal ion. The extent of uptake of divalent metal ions by hematite was in order of Fe> or =Zn>Co> or =Ni>Cd. For all metals tested, there was an instantaneous adsorption followed by a relatively slow stage that continued for the next 1-5 days. This sequence occurred in both single and binary systems, and could have been due to a variety of sorption site types or due to slow conversion from outer- to inner-sphere surface complexes due to increasing surface charge. Sorption competition was observed between Fe(II) and the other metal ions. The displacement of Fe(II) by Me(II) was in order of Ni approximately Zn>Cd, and the displacement of Me(II) by Fe(II) was in order of Cd>Zn approximately Ni>Co. Fixed Fe(II) was in order of Fe+Co (20%)>Fe+Cd (6%)>Fe approximately Zn (4%)>Fe approximately Ni (4%) after 30 days. There was no fixation for the other metals in single or binary systems.

Fate of hydrolyzed Al species in humic acid coagulation

The hydrolysis of Al-based coagulants in acidic conditions is necessary for the removal of organic matter by the coagulation/sedimentation process. However, interactions between hydrolyzed Al species and organic matter are complicated and this makes it difficult to optimize coagulant dosing for organics removal. The goal of this study was to investigate the reactions of hydrolyzed Al species in the coagulation of organic matter. Two polyaluminum chloride (PACl) coagulants, a commercial product with sulfate (PACl-C) and lab-prepared material (PACl-Al13) containing 7% and 96% of total Al as Al13, respectively, have been applied to investigate the coagulation of humic acid (HA). At pH 6, a lower dosage of PACl-Al13 than of PACl-C was required for optimized HA removal through coagulation/sedimentation due to the strong complexation and charge neutralization by Al13. Observation of the coagulation process using wet scanning electron microscopy showed that PACl-C produced both clustered flocs and linear precipitates in the presence of sulfate while PACl-Al13 produced curled precipitates due to the formation of intermolecular complex, when both coagulants were added at the optimum doses. Investigation of Al-HA floc by (27)Al-NMR and Al 2p XPS suggested that monomeric Al (Alm) was hydrolyzed into Al(OH)3 with tetrahedron for PACl-C coagulation while a half of Al13 slowly decomposed into octahedral Al-HA precipitates for PACl-Al13 coagulation. Meanwhile, C ls XPS indicated that aromatic CC of HA was preferentially removed from solution to Al-HA flocs for both PACl-C and PACl-Al13 coagulation. It was concluded that Al-HA complexation strongly affects the reaction pathways for Al hydrolysis and the final nature of the precipitates during PACl coagulation of HA and that the hydrolysis products are also strongly affected by the characteristics of the PACl coagulant.

Modeling the effect of particle size and charge on the structure of the filter cake in ultrafiltration

A force balance model was developed to predict the effects of particle size, particle size distribution and surface potential on the structure of the filter cake. The model predicts that a stable filter cake is formed at low surface potentials and that the filter cake becomes unstable when the surface potential is larger than 30 mV. The model predicts a minimum porosity and permeability for filter cakes formed at intermediate surface potential. The surface potential that produces the lowest porosity increases with increasing particle size in the system. The specific resistance of the filter cake was a function of particle size and surface potential. The surface potential for the maximum specific resistance in the uniform particle system agreed with reports in the literature.

Efficient recovery of nano-sized iron oxide particles from synthetic acid-mine drainage (AMD) water using fuel cell technologies

Acid mine drainage (AMD) is an important contributor to surface water pollution due to the release of acid and metals. Fe(II) in AMD reacts with dissolved oxygen to produce iron oxide precipitates, resulting in further acidification, discoloration of stream beds, and sludge deposits in receiving waters. It has recently been shown that new fuel cell technologies, based on microbial fuel cells, can be used to treat AMD and generate electricity. Here we show that this approach can also be used as a technique to generate spherical nano-particles of iron oxide that, upon drying, are transformed to goethite (α-FeOOH). This approach therefore provides a relatively straightforward way to generate a product that has commercial value. Particle diameters ranged from 120 to 700 nm, with sizes that could be controlled by varying the conditions in the fuel cell, especially current density (0.04-0.12 mA/cm(2)), pH (4-7.5), and initial Fe(II) concentration (50-1000 mg/L). The most efficient production of goethite and power occurred with pH = 6.3 and Fe(II) concentrations above 200 mg/L. These results show that fuel cell technologies can not only be used for simultaneous AMD treatment and power generation, but that they can generate useful products such as iron oxide particles having sizes appropriate for used as pigments and other applications.

Enhancement of fermentative bioenergy (ethanol/hydrogen) production using ultrasonication of Scenedesmus obliquusYSW15 cultivated in swine wastewater effluent

The influence of ultrasonication pretreatment on fermentative bioenergy [ethanol/hydrogen (H2)] production from a newly isolated microalgae biomass (Scenedesmus obliquusYSW15) was investigated. S. obliquusYSW15 biomass was sonicated for 0 min (control), 5 min (short-term treatment), 15 and 60 min (long-term treatment), which caused different states of cell lysis for microbial fermentation. Long-term sonication significantly damaged the microalgal cell integrity, which subsequently enhanced the bioenergy production. The accumulative bioenergy (ethanol/hydrogen) production after long-term sonication was almost 7 times higher than that after short-term treatment or the control. The optimal ratio of microalgal biomass to anaerobic inoculum for higher bioenergy production was 1:1. Microscopic analyses with an energy-filtering transmission electron microscope (EF-TEM) and an atomic force microscope (AFM) collectively indicated that cells were significantly damaged during sonication and that the carbohydrates diffused out of the microalgae interiors and accumulated on the microalgae surfaces and/or within the periplasm, which led to enhanced bioaccessibility and bioavailability of the biomass. These results demonstrate that ultrasonication is an effective pretreatment method for enhancing the fermentative bioenergy production from microalgal biomass.

Reduction of U(VI) by Fe(II) in the presence of hydrous ferric oxide and hematite: effects of solid transformation, surface coverage, and humic acid.

Fe(II) was added to U(VI)-spiked suspensions of hydrous ferric oxide (HFO) or hematite to compare the redox behaviors of uranium in the presence of two different Fe(III) (oxyhydr)oxides. Experiments were conducted with low or high initial sorption density of U(VI) and in the presence or absence of humic acid (HA). About 80% of U(VI) was reduced within 3 days for low sorbed U(VI) conditions, with either hematite or HFO. The {Fe(3+)} in the low U(VI) experiments at 3 days, based on measured Fe(II) and U(VI) and the assumed presence of amorphous UO(2(s)), was consistent with control by HFO for either initial Fe(III) (oxyhydr)oxide. After about 1 day, partial re-oxidation to U(VI) was observed in the low sorbed U(VI) experiments in the absence of HA, without equivalent increase of dissolved U(VI). No reduction of U(VI) was observed in the high sorbed U(VI) experiments; it was hypothesized that the reduction required sorption proximity of U(VI) and Fe(II). Addition of 5mg/L HA slowed the reduction with HFO and had less effect with hematite. Mössbauer spectroscopy (MBS) of (57)Fe(II)-enriched samples identified the formation of goethite, hematite, and non-stoichiometric magnetite from HFO, and the formation of HFO, hydrated hematite, and non-stoichiometric magnetite from hematite.

Nitrite reduction with hydrous ferric oxide and Fe(II): Stoichiometry, rate, and mechanism

Fe(II)/Fe(III) oxide is an important redox couple in environmental systems. Recent studies have revealed unique characteristics of Fe(II)/Fe(III) oxide and reactions with oxidizing or reducing agents. Nitrite was used as an oxidizing agent in this study in order to probe details of these reactions and hydrous ferric oxide (HFO) was used as the Fe(III) oxide phase. Abiotic nitrite reduction is a significant global producer of nitric oxide (a catalyst for production of tropospheric ozone) and nitrous oxide (a greenhouse gas and contributor to stratospheric ozone depletion). All experiments were conducted at pH 6.8 using a strictly anoxic environment with mass-balance measurements for Fe(II). Oxidation of Fe(II) was negligible in the absence of HFO. The reaction was fast in the presence of HFO and was described by d[Fe(II)]/dt=-k(overall)[Fe(II)(diss)][Fe(II)(solid-bound)][NO(2)(-)] (k(overall)=2.59x10(-7)microM(-2)min(-1)) for Fe(II)/Fe(III) molar ratios less than 0.30. The reaction was inhibited for higher Fe(II)/HFO ratios. The concentration of solid-bound Fe(II) was constant after an initial equilibration period and the reaction stopped when dissolved Fe(II) was depleted even though substantial solid-bound Fe(II) and nitrite remained. The results regarding rate-dependence and conservation of solid-bound Fe(II) and inhibition of reaction at high Fe(II)/Fe(III) ratios were similar to our earlier results for the Fe(II)/HFO/O(2) system [Park, B., Dempsey, B.A., 2005. Heterogeneous oxidation of Fe(II) on ferric oxide at neutral pH and a low partial pressure of O(2). Environmental Science and Technology 39(17), 6494-6500.].

Effects of wastewater effluent organic materials on fouling in ultrafiltration.

The objective was to determine the effects of wastewater effluent organic materials (EfOM) on fouling of ultrafilters (100kDa polyethersulfone (PES)). EfOM constituents were sequentially removed, first by removing particles down to the approximate ultrafilter pore size and then by removing dissolved EfOM based on functionality. Particles and colloids >20nm accounted for 19% of total organic carbon (TOC), including 96% of EfOM >100kDa. Removal of particles and colloids resulted in increased fouling, attributed to increased contact of dissolved EfOM with the membrane. Hydrophobic and hydrophilic (HPO/HPI) acids were 22% of total EfOM, and accounted for nearly all of the fouling. HPO/HPI base/neutrals were 59% of EfOM, but did not cause any significant fouling. Although HPO/HPI base/neutrals did not cause any fouling, they were the dominant EfOM constituent at the surface of fouled and then hydraulically cleaned membranes, as measured by attenuated reflectance infrared spectroscopy. Since the filtration runs were short, the effects of HPO/HPI base/neutrals on long-term fouling should be further investigated, but these results cast doubt on the presumption that organic materials that are identified during membrane autopsies are necessarily a primary cause of fouling. These results also indicate that wastewater EfOM should be treated to remove HPO/HPI acids prior to membrane filtration.