Iso-Electric Point and Iron Content in Fe-ASG Geopolymers

Introduction

Among inorganic impurities, some of the greatest threats to human health are associated with the presence of high levels of toxic or heavy metals in drinking water. The sources of contamination and resulting health effects span a wide spectrum of chemical compounds. Some of these inorganic compounds are present due to natural formations in the earth's crust; however, they can also originate from industrial and agricultural waste products that subsequently enter major water systems.

Geopolymers and their engineering applications have attracted significant attention from the scientific community, owing to properties such as good thermal stability and high resistance to aggressive environments. Researchers are currently exploring economically feasible methods for removing heavy metal contaminants. This study was conducted to test an absorbent new media technology with the potential to assist in the removal of such contaminants. The methodology and experimental design were constructed to provide insights into this emerging field of research.

Inorganic particles dispersed in a solution are electrically charged due to their ionic characteristics. When particles are dispersed, they are surrounded by oppositely charged ions; this is referred to as the particle's surface, or first, layer. A second layer is then formed that contains varying compositions of ions. Zeta potential is the term used to describe the electrical potential of these first and second layers surrounding a particle. A zero electrical potential allows particles to aggregate and become unstable, as there is no force to prevent them from coming together. Zeta potential is measured in units of velocity (meters per second). As a general rule, the higher the absolute value of zeta potential — whether positive or negative — the more stable and dispersed the system will be. Geopolymerization is a developing field of research focused on utilizing solid waste and by-products, providing a mature and cost-effective solution for treating and storing hazardous residue under demanding environmental conditions.

Aluminum silicate-based geopolymer (ASG) media is an economically feasible technology suited for water treatment systems and is among the most commonly used adsorbents for the removal of inorganic contaminants (Khale & Chaudhary, 2007). Studies have examined the inorganic material removal capabilities of modified ASG (Duxson et al., 2007). However, the instability of modified ASG media under various pH ranges may result in leaching of the targeted contaminant. By contrast, oxide metals such as iron have proven to be more stable across a wider pH range and could potentially serve as a viable and economically attractive alternative.

Iron hydroxide impregnated ASG is an emerging technology currently under investigation for potential use in water treatment facilities. Initial results have suggested favorable inorganic compound removal capabilities inherent in this new media. The primary objective of this study was to determine the relationship between the iso-electric point and iron content in iron hydroxide impregnated ASG. Examination of the iron content and the aluminum silicate ratio will help determine the tested media's behavior in relation to certain contaminants under different pH water conditions.

It was hypothesized that the iron content in each sample would affect the surface charge of the modified media. Alternating the pH from acidic to basic and recording zeta potential at each pH reading was used to determine the iso-electric point of each modified media. To test the hypothesis, zeta potential tests were conducted on two types of aluminum silicates in their virgin states and eight iron hydroxide impregnated states. An analysis was then performed to determine whether any correlation was present in the zeta potential results.

This study was conducted under the following assumptions: the samples were prepared and collected with accurate amounts of iron composition with aluminum silicate for each sample; the collection equipment was clean and free of residues; the zeta potential instrument was appropriately calibrated and operating without faulty data; the temperature during experiments did not affect the samples; and the modified media was diluted using 10 mM HNO₃ to suit the instrument analyses.

The following limitations apply to this study. First, the samples were prepared and collected by another party. Additional limitations arose from the level of accuracy and sensitivity of the equipment and instruments used. Furthermore, the samples had to be diluted to achieve a suitable particle concentration free of air bubbles in order to conduct the analysis.

Literature Review

Geopolymerization

Silicones are synthetic polymers with unique physical properties, including relatively high melting and boiling points not generally found in other polymers. As a result of their enhanced thermal stability, silicones are inherently more resistant to the effects of heat, cold, sunlight, adverse weather, and common chemical exposure. They have been integrated into thousands of household and medical products — including lubricants, breast implants, adhesives, dishware, and water repellents — and can be found within every major industrial segment of the marketplace (DeLuca, Kaszuba, & Mattison, 2006). During the last two decades, geopolymerization has emerged as a possible technological solution for the effective stabilization and immobilization of toxic materials (Jaarsveld, Deventer, & Lorenzen, 1997). However, the instability of ASG modified media under various pH ranges may result in leaching of the targeted contaminant. By contrast, oxide metals such as iron have proven more stable across a wider pH range and could potentially serve as a viable and economically attractive alternative.

Members of Ain Shams University, the National Authority for Military Productions, and the Atomic Energy Authority in Cairo investigated the various parameters that influence the adsorption of iron and aluminum copolymer impregnated with silica (Enein, Eissa, Diafullah, Rizk, & Mohammed, 2009). The parameters investigated included pH, mass, and agitation. These factors help identify what affects the adsorption and removal of heavy metals such as arsenic. The study concluded that poly-inorganic coagulants based on silica or iron are most effective in removing pollutants from wastewater. Iron and aluminum copolymer impregnated with silica was found to have a maximum adsorption capacity for arsenic of 146 mg/g of media.

Practical Applications

Jang, Shin, Park, and Choi (2003) applied the "wetness impregnation" technique to synthesize and incorporate iron and aluminum oxides into silica media. The purpose of the study was to compare adsorption capacities and kinetics of metal-impregnated SBA-15 media for removing arsenic levels from water. The behaviors of the impregnated media demonstrated greater arsenate adsorption capacities at pH 7.2. The results also suggest that coagulation and fluctuation mechanisms — including oxidation state, adsorption isotherms, kinetics, and structural properties — facilitated by the impregnation technique and compound composition can create a next-step method for removing arsenic from water.

Geopolymers involve the silicates and aluminates of by-products undergoing the process of geopolymerization. This process is environmentally friendly and requires only moderate energy to produce (Nicholson, Fletcher, Miller, Stiling, & Morries, 2005). Literature demonstrates that certain mix compositions and reaction conditions — such as Al₂O₃/SiO₂ ratio, curing temperature, duration, and pH — significantly influence the formation and properties of a geopolymer. Geopolymers are used to manufacture concrete pavements and concrete products, and for the removal of toxic metal wastes in heat- and chemically resistant applications (Nicholson et al., 2005).

The iron content impregnated is significant for determining how surface charge fluctuates under different pH conditions. Studies on geopolymers are mostly based on traditional foundation materials such as metakaolin (Fe₂O₃ content of 2%), fly ash (Fe₂O₃ content of 10%), and blast furnace slag (Fe₂O₃ content of 0.5%) (Stucki, Golden, & Roth, 1984). Studies have indicated that fly ash with a content of 10% is favorable for impregnation into geopolymers (Stucki et al., 1984). Further research has suggested that calcium silica hydrate (CSH), although present at pH 12, was largely unobserved at pH 14; this observation is relevant to the optimal conditions for processing aluminosilicate mineral wastes in large-scale waste-based geopolymerization (Phair & Van Deventer, 2002).

Biological Implications

A study examined Chang liver cells grown in an iron-rich medium for up to twenty weeks under a controlled environment to determine whether any fluctuation in the isoelectric shift occurred due to an acidic shift (Hoy & Jacobs, 1981). Regardless of whether cells were normal or iron-loaded, the researchers found a slight acidic shift in their isoelectric point. This suggests that the acidic ferritins observed on iron loading may be unrelated to subunit composition.

Another study examined the effects of iron concentration during growth on the physicochemical surface properties of colonial variants of Neisseria gonorrhoeae, assessed by aqueous two-phase partitioning in a dextran-polyethyleneglycol system containing positively charged trimethylamino-polyethyleneglycol or hydrophobic polyethyleneglycol-palmitate (Magnusson, Kihlstrom, Norqvist, Davies, & Normark, 1979). This study suggests that the complex effects of iron, combined with other variables known to affect surface charge and hydrophobicity, provide clues about gonococcal surface properties important in promoting virulence.

A further experiment used Ferguson analysis as a method for simultaneously measuring the size and zeta potential of gold nanoparticle-DNA (Au NP-DNA) conjugates. This approach was determined to be highly suitable when particle size is less than 20 nm and biomolecular conformation results in a non-spherical or non-linear conjugate, causing conventional dynamic light scattering and zeta potential measurements to yield inconsistent results (Park & Hamad-Schifferli, 2008).

Methodology

This chapter describes the properties of the media under investigation and details the experimental and analytical procedures. It also provides the approach taken for modeling the iso-electric point of each media type. All glassware, the pH meter, electrodes, and equipment used were cleaned according to standard protocol (see Appendix A).

The Zeta PALS (Phase Analysis Light Scattering) instrument measures surface charge using phase analysis light scattering and determines the electrophoretic mobility of charged colloidal suspensions in an electrical field. Particles move toward the electrode of opposite charge; their velocity is measured and expressed in unit field strength as their mobility. Measurements are typically conducted in a conductive liquid such as potassium nitrate (KNO₃) at different pH values. Avoiding dust and air bubbles is essential; it is therefore necessary to filter the dispersant solution used for preparing the various dilutions and to wash all vessels thoroughly (Brookhaven Instruments Corporation, 2011).

Materials

The Fe-ASG media was previously synthesized and obtained for this investigation. The aluminum silicate ratio and iron content for the virgin and FeO₂-impregnated ASG samples are presented in Table 3.1 below.

Table 3.1: ASG & Fe-ASG Media Tested in This Study

SBIR 3 — Fe content: 10.00%
SBIR 4 — Fe content: 13.50%
SBIR 5 — Fe content: 19.60%
SBIR 6 — Fe content: 19.90%
SBIR 7 — Fe content: 17.00%
SBIR 8 — Fe content: 16.20%
SBIR 9 — Fe content: 16.00%
SBIR 10 — Fe content: 11.77%

A laboratory technician prepared 10 mM concentrations of potassium nitrate (KNO₃) to dilute the Fe-ASG media for each experimental run. In addition, 1 molar and 0.1 molar solutions of both nitric acid (HNO₃) and potassium hydroxide (KOH) were prepared to correct the pH of the media. Buffers of pH 4, 7, and 10 were provided to calibrate the pH meter. A magnetic stir bar was used to keep the diluted media mixed and prevent particle settling.

Other equipment included disposable plastic cuvettes for sampling the suspension, disposable plastic pipettes for liquid samples and acid/base additions, a 100 mL graduated beaker to hold the sample during mixing, a pH meter to read the adjusted sample pH, and a specialized electrode for zeta potential measurements.

Experimental Methods for Zeta Potential Analysis

A total of ten media samples were analyzed: eight impregnated with FeOOH and two untreated ASG controls. Each sample run was prepared using a 100 mL graduated beaker containing 10 mL of media diluted with KNO₃ to a total volume of 100 mL. The diluted media in the beaker was kept on a stirring plate at rate 10 with a magnetic stir bar to maintain particle distribution and ensure adequate mixing when HNO₃ or KOH was added. Each acid and base solution had its own disposable pipette to gradually adjust the pH by adding drops until the desired value was reached.

The pH meter was calibrated using buffers of pH 4 and 7 for acidic measurements, and re-calibrated using buffers of pH 7 and 10 for basic pH measurements. The pH meter was rinsed with nanopure water and gently dried using Kimwipes. Electrodes used for surface charge readings were soaked in nanopure water in a cuvette for at least 24 hours before running the analysis.

After the desired pH was reached, samples were pipetted into a cuvette using a disposable pipette, taking care to ensure no air bubbles formed, before inserting the electrodes into the sample cuvette. The cuvette and electrodes were rinsed with nanopure water three times before adding the adjusted sample. The cuvette was also rinsed with the new adjusted sample before final filling, to normalize it to the new sample pH. A standard deviation of ±5 mV is considered good, and a standard deviation of ±10 mV is acceptable (ZetaPals Protocol v1, 2011).