Ferrous Sulfate

The environmentally beneficial form of chromium is the immobile, nontoxic trivalent chromium. Reduction usually involves the use of reductant or reductant generating material for the purpose of transforming toxic compounds to nontoxic or less toxic ones. However, there are a number of reducing agents such as sulfite, sodium dithionite, hydrogen peroxide and ferrous ion that can be used to achieve this transformation. But ferrous ion has been popularly used possibly because it suites natural environmental process—Fe (II) also is produced through photochemical reactions that occur in sunlit natural waters [9]. Ferrous ion effectively interacts with solid-phase and dissolved-phase Cr (VI) transforming it to Cr (III). Ferrous ion, Fe (II) is oxidized to Fe (III) by releasing the electron while Cr (VI) is reduced by accepting the electron. Therefore, three moles of Fe (II) are required under ideal conditions to reduce one mole of Cr (VI) to Cr (III) as illustrated below (Equations (1)-(3))

F²⁺ →F³⁺ + e⁻ (E° = −0.77 V)

3Fe²⁺ →3Fe³⁺ + 3e⁻

Cr₂ O^2-₇ − +14H⁺ + 6e⁻ →2Cr³⁺ +7H₂O

In the dichromate form, Cr₂ O^2-₇, there are two atoms of Cr (VI). Therefore, six electrons are required to reduce the predominant form of chromium to Cr (III). In which case six atoms of Fe (II) ions are oxidized.

Ferrous Sulfate is also used to catalyze Hydrogen Peroxide. Fenton’s reagent.  Fenton’s reagent uses hydrogen peroxide in the presence of ferrous sulfate to generate hydroxyl radicals that are powerful oxidants. The reaction is fast, releases oxygen and heat, and can be difficult to control when high strength peroxide is used. Because of the fast reaction, the area of influence around the injection point is small. In conventional application, the reaction needs to take place in an acidified environment, which generally requires the injection of an acid to lower the treatment zone pH to between three and five. The reaction oxidizes the ferrous iron to ferric iron and if the subsurface pH is not acidic causes it to precipitate, which can result in a loss of permeability in the soil near the injection point. Over time, the depletion of the ferrous ion can be rate limiting for the process. Chelated iron can be used to preserve the iron in its ferrous state at neutral pH, thus eliminating the acid requirement. The byproducts of the reaction are relatively benign, and the heat of the reaction may cause favorable desorption or dissolution of contaminants and their subsequent destruction. Heat also may cause the movement of contaminants away from the treatment zone or allow them to escape to the atmosphere. Therefore, there are safety concerns with handling catalyzed hydrogen peroxide on the surface, and the potential exists for violent reactions in the subsurface. In many cases, there may be sufficient iron or other transition metals in the subsurface to eliminate the need to add ferrous sulfate.

Arsenic is introduced into the aquatic environment from both natural and manmade sources. Typically, however, arsenic occurrence in water is caused by the weathering and dissolution of arsenic bearing rocks, minerals, and ores. Although arsenic exists in both organic and inorganic forms, the inorganic forms are more prevalent in water and are considered more toxic. Total inorganic arsenic is the sum of particulate and soluble arsenic. A 0.45-micron filter can generally remove particulate arsenic.

Soluble, inorganic arsenic exists in either one of two valence states depending on local oxidation reduction conditions. Typically groundwater has anoxic conditions and arsenic is found in its arsenite or reduced trivalent form [As(III)]. Surface water generally has aerobic conditions and arsenic is found in its arsenate or oxidized pentavalent form [As(V)].

Arsenic is a naturally occurring metalloid that is very mobile in the environment. Its mobility largely depends on the parent mineral form, oxidation state, and mobilization mechanisms, In terms of oxidation state, arsenic can exist in four forms, which are arsenite (As(III)), arsenate (As(V)), arsenic (As(0)), and arsine (As(III)). Among these four arsenic species, the most prevalent forms, which are commonly found in water, are the inorganic arsenite and arsenate .

Because of slow redox transformations, arsenite and arsenate are present in both reduced and oxidized environments. However, under anoxic reducing conditions (e.g., subsurface waters, reduced sediments), arsenic primarily exists as arsenite, whereas arsenate is prevalent in aerobic oxidizing environments, such as surface waters. The pH also plays an important role in determining the state of arsenic. Given a particular pH and redox potential, the speciation of arsenic, including its oxidation state, can be determined through this diagram. This information is particularly useful in the determination of arsenic toxicity, taking the fact that the different arsenic oxidation states possess different toxicities into account. Moreover, considering the fact that negatively charged arsenate (i.e., H2AsO4− and HAsO42−) is generally much easier to remove compared to uncharged arsenite (i.e., H3AsO3).

 

Ferrous sulfate for soil remediation:

1. Heavy Metal Immobilization: Ferrous sulfate is often used in the treatment of soils contaminated with heavy metals, such as lead, cadmium, arsenic, and chromium. When introduced into the soil, ferrous sulfate reacts with these heavy metals to form insoluble compounds, effectively immobilizing them and preventing their uptake by plants or leaching into groundwater. This makes the heavy metals less bioavailable and less toxic.
2. Acidic pH Adjustment: Ferrous sulfate can also help to adjust the pH of highly alkaline soils. The iron(II) ions can react with hydroxide ions, resulting in a decrease in soil pH. This is beneficial in certain remediation scenarios where an acidic pH is desired, such as enhancing the activity of certain soil microbes or improving conditions for plant growth.
3. Sulfate for Sulfate-Reducing Bacteria: The sulfate ion from ferrous sulfate can serve as an electron acceptor for sulfate-reducing bacteria. These bacteria can participate in the biodegradation of organic contaminants, making ferrous sulfate a potential enhancer of bioremediation processes in anaerobic conditions.
4. Cost-Effective: Ferrous sulfate is relatively inexpensive compared to other remediation agents, making it a cost-effective option for large-scale soil remediation projects.
5. Availability: Ferrous sulfate is widely available and is easy to handle and apply, which are practical benefits in soil remediation efforts.

Ferrous sulfate for water remediation:

1. Coagulation and Flocculation: Ferrous sulfate is commonly used as a coagulant in water treatment. It reacts with water to form insoluble iron hydroxides, which can entrap and adsorb suspended particles, including bacteria, algae, viruses, fungi, minerals, and other particles. These formed particles, known as flocs, can then be removed through sedimentation, filtration, or flotation methods.
2. Phosphate Removal: Ferrous sulfate is also effective in removing phosphate from water bodies, a major concern in eutrophication. Iron ions react with phosphate to form insoluble iron phosphate, which can then be removed through sedimentation.
3. Heavy Metal Removal: Ferrous sulfate can be used to remove heavy metals from water. Similar to the process with phosphate, the iron ions can react with certain heavy metal ions to form insoluble compounds, which can then be removed from the water.
4. Reduction of Chromate: Ferrous sulfate can reduce hexavalent chromium (Cr(VI)), a toxic and carcinogenic form of chromium, to trivalent chromium (Cr(III)), a less toxic and less soluble form. This reduced form can then precipitate out of solution, making it easier to remove.

 

Int J Environ Res Public Health. 2016 Jan; 13(1): 62, Published online 2015 Dec 22. doi:  10.3390/ijerph13010062

ATSDR Public Health Statement for Arsenic. Sep 2000

U.S. EPA. Arsenic Treatment Technologies for Soil, Waste, and Water. EPA 542-R-02-004, 2002.

Jude O. Ighere, Karina Honjoya, Ramesh C. Chawla, Chemical Engineering Department, Howard University, Washington DC, USA, December 2014.