Low Cost Adsorbents for Water Treatment

For several years MSE Technology Applications, Inc. has studied the material ApatiteII™ (Patent Number 6,217.775; PIMS NW, Inc.; Drs James Conca and Judith Wright, developers) for the removal of metals, metalloids and radionuclides. Apatite II™ is a form of apatite that appears to have higher reactivity than natural apatite and is lower cost than commercially available bone char. To date our work has shown that ApatiteII™ is very effective in removing uranium.

Several mechanisms for uranium removal by apatite are possible: uranium exchange for the calcium anions in the apatite matrix, non-specific adsorption to the apatite material, or the dissolution of apatite will create condi-tions at the boundary layer that exceed the Ksp for autunite (Ca(UO2)2(PO4)2 _ 10H2O) and this U-bearing crystalline phase will form. Laboratory batch studies observed the presence of meta-autunite (Ca(UO2)2(PO4)2 _ 6H2O) within the apatite material, suggesting the formation of autunite.

Meta-autunite formation was due to the drying step before analysis by X-ray Powder Diffraction.Additionally, we have investigated the removal of soluble cadmium from syn-thetic and authentic contaminated groundwater. Cadmium removal by apatite is probably due to one or all of three mechanisms: ion exchange, nonspecific adsorption, and pre-cipitation of cadmium phosphate minerals.

Previous studies have observed a cadmium phosphate mineral, otavite. However, the removal of cadmium only occurs at about 5 wt.% of the apatite added, as opposed to 10 to 20 wt.% in the case of uranium and lead. initial studies with this material showed that uranium was rapidly removed from solution in batch shake flasks. Bacterial growth was associated with the incubation of this material at room temperature however, sterilized controls vs. unsterilized flasks did not show significant difference in the rate of uranium uptake by the apatite.

Subsequently, batch studies were undertaken to determine whether surface area had an effect on uranium removal. Previous studies had used a very fine (surface area = 2.75 m 2 /g) powdered apatite that would not be as easily implemented in a canister or barrier situation in the field. Various crushed sizes were tested and it was shown that for the range tested (0.425 to 4.75 mm) there was no significant difference in the rate of uranium uptake.The object of these studies was to follow up the batch studies with column studies to determine if the apatite material could be used in permeable reactive barrier systems or canister type configurations for removal of uranium and cadmium from contaminated groundwater.

Most recently we have begun studies to determine if apatite can be added in a soil-mixing scenario to remove metals from contaminated soils. Batch and column studies have been performed by MSE to determine the efficacy of metal cation removal from aqueous solutions by an organic apatite material. results have shown that the Apatite II™ media rapidly removes uranium (C0=5mg/L)from solution to below detection levels, and moderately removes cadmium.

The mechanism of uranium removal appears to be by reprecipitation as the uranium phosphate autunite. Cadmium removal appears to be via several mechanisms including adsorption and phosphate mineral precipitation. Currently, a soil-mixing study is being carried out to test immobilization of metal cations in contaminated soils.

2. HEAVY METAL REMOVAL BY ZERO-VALENT IRON

Zero-valent iron Fe(0) has been investigated as a potential remediation agent for the removal of a whole series of heavy metals. Possible removal mechanisms, depending on the metal of interest, are reduction, cementation, surface complexation and (co)precipitation The basis for any of these reactions is the corrosion of iron. The first corrosion product is amorphous ferrous hydroxide, which is predicted thermodynamically to convert to magnetite (Fe3O4).

Mixed valent iron salts, known as green rusts may also form. Their subsequent oxidation can lead to the formation of magnetite, maghemite, goethite and lepidocrocite. As a result of these reactions, the iron surface is coated by a layer of iron oxides and oxyhydroxides, similar to natural oxide solid phases, where heavy metals may interact.

The chemistry of metal ions in natural waters strongly depends on the formation of complexes. Complexes reduce the concentration of the free metal species in solution, and affect the solubility, the toxicity, and the mobility of the metal.

Polluted groundwaters, especially in the case of landfill leachate contaminated groundwater, contain elevated concentrations of inorganic and organic ligands Three possible scenarios in aqueous systems containing both humic acids and heavy metals, in contact with hematite. Firstly, the binding of humic acids to the oxide surface may block the binding sites and compete with metal removal.

Alternatively, complexes between metals and humic acids may remain in solution and prevent the metal from adsorbing to the oxide. Thirdly, metal adsorption can be enhanced by the formation of ternary humic acids-metal-surface complexes. The enhanced metal binding can result from the adsorption of metal-humic acid complexes, or through metal complexation by the sorbed humic acids. The overall effect from addition of humic acids on metal binding depends on the stability of the numerous interactions involved.

The results of this study indicate that zero-valent iron is a promising reactive agent for the in-situ removal of mixed heavy metals (i.e. Ni, Zn and Cr(VI)) from contaminated groundwater. Humic substances bind heavy metals and influence their fate and mobility. We found that humic acids formed metal-humate complexes remaining in solution, which prevented the removal of Zn and Ni in batch kinetic experiments.

Chromate removal was not affected. Sorption of humic acids to the iron surface hindered the metal removal reactions in a longer-term column test, especially for Ni, probably through blocking of reactive sites.

Humic acids impact the efficiency and lifetime of a reactive iron barrier for in-situ removal of heavy metals. The presence of dissolved organic matter in groundwater should therefore be considered in the design of a permeable barrier.

A laboratory study was set-up to investigate the effects of Aldrich humic acids on the removal of zinc, nickel and chromate in zero-valent iron systems. The rate and the extent of zinc and nickel removal were negatively impacted by the presence of the humic acids in batch kinetic experiments. Chromate removal was not affected. It was hypothesized that the formation of humic acid-heavy metal complexes prevented the removal reactions at the iron surface. Two parallel column systems were set-up with the mixed heavy metals (5 mg/l each).

The first system acted as reference without humic acids, while the second system was fed with humic acids (20 mg/l). Both column systems efficiently (> 90%) removed the heavy metal mixture during the first 27 weeks. When the input heavy metal concentration was increased to 8-10 mg/l, a significant breakthrough of nickel and zinc occurred in the column system with humic acids.

Conversely, chromate and humic acids did not significantly break through. The accumulated humic acids on the iron surface (approx. 5 mg/g) probably hindered the nickel and zinc removal.

After 60 weeks, the effect of humic acids on leaching of the accumulated metals (approx. 2 mg/g) was investigated. No significant leaching was observed. The results of present study indicate that the impact of dissolved organic matter on the efficiency and lifetime of a reactive iron barrier for in-situ removal of heavy metals should be considered in the design of the barrier.

3. SURFACTANT-MODIFIED CLAYS

Bentonite and montmorillonite clays are hydrophilic due to the hydration of inorganic cations existing in the interlayers of clay. As a result, these clays are not effective sorbents to remove the NOCs in aqueous system. Organoclays can be prepared by ion-exchanging inorganic cations on the clay surface for organic cations with long hydrocarbon chains such as hexadecyltrimethylammonium (HDTMA) cation.

The intercalated organic cations may expand the interlayer of clay and form pseudo-organic phase that increases the sorption capacity of NOCs from water. Organoclays can provide a wide range of applications in wastewater treatment and permeable reactive barrier . Contaminant immobilization using organoclay coupled with in situ biodegradation would provide a comprehensive restoration technology to permanently eliminate target organic contaminants Several studies have been reported on the sorption of ionizable organic compounds (IOCs) in organoclays (for example, Zhu and Chen, 2000).

However, no study has been reported on desorption of IOCS from organoclay. IOCs can exist as either protonated ordeprotonated species depending on pH of the solution. sorption isotherms of pentachlorophenol (PCP) onto HDTMA-clay at two pHs, 5.5 and 10, were nonlinear and nearly identical, regardless of the speciation of PCP. Stapleton et al. (1994) studied the sorption of PCP onto HDTMA-clay at pHs from 4 to 8.5. They concluded that sorption of IOCs on HDTMA-clay could be affected by their speciation.

In this work, natural bentonite modified with the HDTMA cation to the extent of 100% of the cation exchange capacity (CEC) of the bentonite (100% HDTMA-bentonite) was used as sorbent. Single- and bisolute sorption (desorption) of phenol and 4-chlorophenol (4-ChP) were carried out in a batch-type adsorber. Single-solute sorption (desorption) data were analyzed using Freundlich model. Bisolute competitive sorption desorption) data were compared with the predictions from the ideal adsorbed solution theory (IAST) coupled with the Freundlich single-solute model (Freundlich-IAST model).

In parallel, the effect of pH on the sorption and desorption of chlorinated phenols (2,4-DChP and 2,4,5-TChP) in 100% HDTMA-montmorillonite clay was investigated at two different pHs (4.85 and 9.15). Sorption and desorption behaviors over a wide range of solute concentration were investigated by performing sequential sorption until sorbed chemical concentration deviates from the saturation. Effect of competition between solutes on sequential sorption and desorption was also investigated. Bisolute competitive data were compared with the predictions from the Freundlich-IAST model.

As far as authors know,this is the first attempt on modeling of sequential competitive sorption and desorption of IOCs in organoclay. Results of this study may provide a valuable insight into the development of remediation strategies for the IOCs-polluted groundwater using the organoclay as a permeable reactive barrier (PRB).Single- and bisolute competitive sorption and desorption of phenols in natural clays modified with hexadecyltrimethylammonium (HDTMA) were investigated.

Two different types of experiment were conducted: (1) single-step sorption/desorption of phenol and 4-chlorophenol (4-ChP) in HDTMA-bentonite and (2) sequential sorption/desorption of 2,4-dichlorophenol (2,4-DChP) and 2,4,5-trichlorophenol (2,4,5-TChP) in HDTMA-montmorillonite. As expected by the Kow, 4-ChP (log Kow = 2.39) exhibited higher sorption affinity and desorption-resistance than phenol (log Kow = 1.46).

This is mainly attributed to stronger hydrophobic interactions between the solute and the pseudo-organic medium formed by HDTMA cation. Sorption and desorption isotherms were nearly identical indicating that sorption of phenols in organoclay mainly occurs via partitioning into the core of the pseudo-organic medium, thereby causing desorption nearly reversible.

In the bisolute system, competition between the solutes reduced sorption amount of each solute. The ideal adsorbed solution theory (IAST) coupled to a single-solute Freundlich model successfully predicted the bisolute competitive sorption/desorption equilibria. Effect of pH on the sequential sorption/desorption of 2,4-DChP and 2,4,5-TChP in HDTMA-montmorillonite was investigated. For both chlorophenols, neutral species at pH 4.85 exhibited higher sorption affinity and desorption-resistance than anionic species at pH 9.15.

Desorption of chlorinated phenols was nearly reversible at both pHs and strongly dependent on pH of the aqueous phase, regardless of their speciation (neutral or anionic) during sorption stage. The IAST coupled with single-solute Freundlich model successfully predicted bisolute sequential sorption and desorption equilibria.

4. CARBONS PREPARED FROM BAMBOO

Most bamboos contain large amounts of ligneous fiber and can therefore be carbonized into chars, which can be used in decoration, in purifying drinking water, for indoor air filtering, dehumidifying, thermal insulation, electromagnetic wave shielding, etc. However, bamboo is rarely used as the raw material for activated carbon.

Meso bamboo is grown profusely in Taiwan. Its stem diameter can reach up to 15 cm. In earlier days, it was commonly used for building, furniture, eating and cooking utensils, foods, and food processes. Lately, there only use that seems to have remained, is that of using bamboo shoots for food. It is regrettable that the mature bamboo does not seem to have many uses any more. Since the utility value of bamboo has greatly dropped, bamboo-growing has mostly been abandoned reducing the number of plants. The reduction in the number of bamboos grown has caused severe ecological damage to the environment and to water-soil conservation.

Bamboo grows fast, absorbing, CO2 from the atmosphere at great speed, so it effectively helps to slow down global greenhouse effect. In previous studies, oak, bamboo, coconut shell, and cedar were activated with steam to obtain activated carbons. The results showed that physical properties (BET surface area and pore volume) and adsorption capacities (chloroform adsorption) of the activated carbons derived from bamboo were lower than those from the other three raw materials Furthermore, studies have shown that of the carbons derived from bamboo dust, coconut shells, groundnut shells, rice husks, and straw, straw carbonhas the highest adsorption capacity being 5.9 times that of bamboo dust carbon, which is the lowest.

From this evidence it is clear that it is not easy to make good activated carbon from bamboo. The aim was to prepare porous carbons from Meso bamboo using KOH etching and CO2 gasification processes. The physical properties of the carbons, namely the BET surface area, pore size distribution, and the total pore volume were compared. Their capacities for the adsorption of basic blue 1, methylene blue, p-cresol, p-chlorophenol, p-nitrophenol, and phenol from water were systematically investigated.

Meso bamboo char was removed, crushed, and sieved to a uniform size ranging from 0.83 to 1.65 mm. These powders were well mixed with water and KOH in stainless steel beakers with water/KOH/char weight Meso bamboo was first carbonized for 1.5 h at 450 o C, then soaked in KOH solutions with KOH/char ratios of 0.5, 1, 2, and 3. Three kinds of activation processes, no, low, and high CO2, were used. Carbons activated under high CO2 conditions exhibited high BET surface areas ranging from 1627 to 2444 m 2 g -1 .

The adsorption of methylene blue, basic brown 1, p-nitrophenol, p-chlorophenol, p-cresol and phenol from water at 30 o C on the activated carbons was studied. All adsorption equilibrium isotherms were in agreement with the Langmuir equation, and were used to compare the adsorption of covered areas (Sc/Sp) of the activated carbons with the different combinations of CO2 gasification and KOH/char ratios. In this research, activated carbons with high surface area were obtained from Meso bamboo having excellent adsorption capacity for dyes and phenols ratios of 1/0.5/1, 1/1/1, 2/2/1, and 2/3/1.