NANOCOMPOSITE MATERIALS FOR ARSENIC REMOVAL FROM CONTAMINATED WATER AND COMPOSITIONS THEREFROM TECHNICAL FIELD OF THE INVENTION

This invention relates to the use of nanomaterials from agricultural byproducts, particularly modified nanosilica, for arsenic remediation and removal from groundwater, and specifically to the methods of production of these materials.

BACKGROUND OF THE INVENTION

Arsenic can be found in soil, water and air (Hughes et al, 2011). The occurrence of arsenic in air can be attributed to some anthropogenic activities such as mining and smelting. Such activities may be also considered as the major cause of arsenic contamination in water. Mining activities produce tailings that may contain heavy metals like lead, cadmium and arsenic. Exposure to this toxic metal may also come from food and medicines. Human exposure may be through inhalation, ingestion and dermal contact (Singh et al., 2007).

Nanotechnology, which acts by adsorption and chemical precipitation .processes have the potential to address environmental concerns such as arsenic removal and are now being explored due to its high efficiency and low cost (Ahmed, 2011). Compared to conventional technologies that may have been developed for the detection and removal of arsenic compounds, the nanoscale versions are expected to be more sensitive and efficient as a consequence of their small sizes and greater surface areas. Reducing the amount of arsenic in a given water system down to the maximum contamination limit {10 ppb) requires an adsorbent that has strong affinity for both As3+ and As5+ . The adsorbent should also have high surface area and accessible pores in order to facilitate effective arsenic adsorption (Sun et al., 2012). Zeolites, aluminosilicates with well-defined pore structures, have long been used in a variety of applications including particularly the ability to remove heavy metals, including arsenic, in groundwater. Rice hull ash has been utilized in the synthesis of zeolites due to the increasing popularity of indigenous materials as a green raw materials for zeolite synthesis as the material is known to have high silica content (Sari et al., 2009; Chiarakom et a/., 2006). The exterior of rice husk is mostly made up of silica. The composition of rice husk is 40-500;6 cellulose, 25- 30% lignin, 15-20% ash and 8-15% moisture. Combustion of rice hull will produce white ash which contains 87-97% silica (Yalcin, 2001 ). Other by- products such as fly ash and paper sludge ash have also been utilized for zeolite synthesis (Wang et al, 2007; Querol et al., 1999; Wajima et al., 2006). Chitosan obtained from the deacetylation of chitin derived from the exoskeleton of crabs and shrimps is now being studied as a potential adsorbent of arsenic and other heavy metals. Over 150 zeolite types have been synthesized in addition to the 48 naturally occurring ones. Moreover, the range of its uses can be extended by incorporating various metal cations such as titanium, zinc or iron, varying the nature of the structure directing agents or varying the ratios of the precursor materials as well as the conditions for their preparation. Because of the presence of well-defined pores and channels, these materials are excellent host for gases, ions and organic molecules and can be used for environmental decontamination (Davis, 1992; Baelocher and Meier, 2001). Studies have shown that the addition of aluminum does not significantly increase the affinity of rice hull ash nanosilica for arsenic, on the other hand, addition of iron significantly improved the ability of the nanosilica to remove arsenic from contaminated water. Thus, use of iron-modified nanosilica from rice hull ash for arsenic removal from contaminated water addresses two problems in farming communities: (a) access to cleaner, more potable water; and (b) disposal of a large volume of rice hulls and rice hull ash that would otherwise be treated as waste rather than as a low-value agricultural by-product that can be transformed to high value industrial products, such as water filters, adsorbents for environmental remediation and as components for nanosensors.

Hydrogels may also be used for encapsulating and removing environmental pollutants. Hydrogels are superabsorbent, non-water-soluble polymers that are quite responsive to environmental stimuli such as pH, temperature, ionic strength, magnetic field or specific chemicals. Although current applications are largely in the area of drug delivery because of the ability to encapsulate large molecules, emerging applications include biosensors and materials for environmental cleanup. Technologies such as US201714491 1 (‘911) and CN106111071 (‘071) have made use of silica materials and modified silica derivatives to purify waters contaminated with heavy metals. ‘911 , in particular, has produced a sulfydryl modified magnetic mesoporous Si02 to reduce cadmium in waste water. CN 105540726 (726) on the other hand discloses a method for removing pentavalent arsenic from wastewater by adopting a magnetic chitosan/biochar composite material. These technologies however failed to disclose how to convert agricultural by-products such as rice hull ash to nanomaterials that will be utilized for the purification of water contaminated with heavy metals.

SUMMARY OF THE INVENTION

The present invention describes a process of producing iron-modified nanosilica powder and iron-modified nanosilica aerogel beads to be used for removing arsenic from water. It makes use of rice hull as its source of nanosilica. The isolated silica was then modified to increase its affinity for arsenic. DETAILED DESCRIPTION OF THE INVENTION

The process for producing the iron-modified nanosilica powder or beads is divided into two major stages - the preparation and purification of nanosilica from rice hull, and the preparation of iron-modified nanosilica powder or beads.

Preparation and Purification of Nanosilica from Rice Hull

Rice hulls were ground using a Wiley Mill. The ground rice hulls were mixed with 1 N HCI at a ratio of 1 :5 (wt. of rice hulhvol. of acid). The mixture was then heated with occasional stirring at 60 °C for about 4 hours. The mixture was cooled to room temperature, then filtered. The residue was washed with deionized water until the washings became neutral to litmus, then the residue calcined in a muffle furnace at 6500°C for six hours. The residue obtained is the nanosilica. The nanosilica was then dissolved in 2.5N NaOH at a ratio of T.4 (wt. of nanosilica: vol. of base). The resulting silica sol was filtered in order to remove the undissolved particles. Nanosilica was reprecipitated using 2.5N HCI while stirring to ensure that the generated silica is nano-sized. Preparation of Iron-modified Nanosilica Powder

The iron source (ferric sulfate or ferric chloride) was first dissolved in 2.5 N NaOH. The excess iron was then filtered out using a Buchner funnel with a Whatman#4 filter paper. The purified nanosilica sample was then added to the alkaline solution of the iron salt and the resulting sol was stirred for 10 hours. It was then titrated with 5N H2SO4 to reduce the pH to 7.5 - 8.5. The resulting mixture was then filtered through a Buchner funnel using a Whatman#4 filter paper. The residue was then washed with deionized water. It was air dried, then oven dried at 120 °C for 12 hours. Preparation of Iron-Modified Silica (Fe-Si) Beads

A sample of nanosilica powder was dissolved in 2.5 N NaOH solution. The resulting solution was used to titrate a sample of 5 M acetic acid until pH 4.0. The resulting mixture was filtered and the filtrate collected. The iron source was then added to the solution. This solution of iron modified silica sol was slowly dropped to form beads in a column containing ammonium hydroxide with surfactant as the aqueous layer and hexane as the organic layer. Iron modified silica beads were collected, washed, and then aged in ammonium hydroxide for two hours before washing it again with de-ionized water, then with acetone or isopropyl alcohol The beads were air dried then oven dried. Nanosilica beads are placed in cartridges then installed/attached to a water source

The preferred embodiment of this invention is described in the above-mentioned detailed description. It is understood that those skilled in the art may conceive modifications and/or variations to the embodiment shown and described therein. Any such modifications or variations that fall within the purview of this description are intended to be included therein as well. Unless specifically noted, it is the intention of the inventors that the words and phrases in the specification and claims be given the ordinary and accustomed meanings to those of ordinary skill in the applicable art. The foregoing description of a preferred embodiment and best mode of the invention known to the applicant at the time of filing the application has been presented and is intended for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and many modifications and variations are possible in the light of the above teachings.