Iron Removal Strategies
Iron, usually presents in groundwater as divalent ion (Fe2+) and is considered as source of membrane scaling.
The main target in our case study is the removal of iron in groundwater before passing through reverse osmosis membranes as pretreatment technique to avoid membrane fouling.
Take in account that the antiscalant feeding before membranes is effective with respect to precipitation.
It reduces iron concentration from 3.8 mg/l to 3.12 mg/l, but this iron level is still the main source of membrane problems.
In this case, various treatment methods have been employed to enhance water quality by removing iron.
Alternative processes have been proposed in order to facilitate the operation and to allow the removal of high amounts of iron in the presence, or absence, of dissolved organic matter.
In both cases, a pH adjustment is necessary to maintain iron in the dissolved state to avoid membrane fouling.Ferrous iron is oxidized in air according to the following reaction:
Fe2+ + (1/4) O2+ H+ ”! Fe3+ + (1/2) H2 O …(1)
Potassium Permanganate and Depth Filtration
Conventional treatment for iron removal from groundwater consists of oxidation and depth filtration.
Oxygen or stronger oxidants, such as potassium permanganate (KMnO4), are generally used for Fe 2+ oxidation.
The solid products of oxidation (FeOOH.H2O) are then filtered through a granular bed, commonly green sand19.
The potassium permanganate dose applied must be carefully controlled to minimize any excess passing into supply which could give a pink color to the water.
Potassium permanganate oxidation tends to form a colloidal precipitates which may not be well retained by the filters.
Chlorine and Depth Filtration
The removal of iron along with chlorination step and appropriate dose of chlorine will be discussed.
In particular membrane fouling caused by oxidized particles, was assessed in depth with visualization of the membrane surfaces.
the removal efficiency of dissolved iron increased very rapidly and reached nearly 100% within 20 minutes with the appropriate dose of chlorine, 2.75 mg/L.
With a higher dosage of chlorine 2.75 mg/L, there was no significant increase in the removal of metal ions but more serious membrane fouling occurred.
The use of chlorine may be inadvisable when treating waters containing organic substances due to the possibility of disinfection byproducts (DBPs) formation.
An alternative filter media is manganese greensand20, formed by treating greensand (glauconite), which is a sodium zeolite, with manganous sulphate followed by potassium permanganate.
Mn-greensand removes soluble iron by a process of ion exchange, frequently with the release of hydrogen ions.
The process is therefore pH dependent, being virtually ineffective below pH 6.0 and very rapid at pH values above 7.5.
When the Mn-greensand is saturated it is regenerated by soaking the filter bed with weak potassium permanganate solution.
This procedure oxidizes iron on the surface of Mn-greensand thereby reactivating the exchange sites.
It is reported that the exchange capacity is 1.45 g of Fe /l of Mn-greensand and that 2.9 g of potassium permanganate (as a 1% w/v solution) per liter of Mn-greensand is required for regeneration21.
Alternatively, potassium permanganate is continuously applied to the bed by dosing it at filter inlet, which maintains Mn-greensand active and catalyses the oxidation reaction.
Mn-greensand then acts as a filter medium in addition to catalytic
oxidation of any residual soluble manganese and is usually capped with a layer of anthracite to achieve longer filter runs.
capacity is exhausted will reduce its service life and may cause stainOperating the bed after oxidation
Oxidation and Microfiltration
This treatment is similar to the conventional one except that depth filtration is replaced by microfiltration (MF).
The expected advantage of this treatment is to have a compact separation unit which produces high quality water from a wide range of raw water quality.
In the present study the MF of iron oxide suspensions is removed
Finally, under certain conditions, the presence of free chlorine and other oxidizing agents, in the oxidation processes, will cause premature membrane failure.
Since oxidation damage is not covered under warranty, FilmTec recommends removing residual free chlorine and other oxidizing agents by another suitable pretreatment prior to membrane exposure24.
Ion exchange resin
Ion exchange resins are able to remove many inorganic metal ions from groundwater including iron.
In this case, Amberlite IR120Na, strong acid cation exchanger was used,
Ion exchanger was carried out in a vessel constructed of a fiberglass reinforced vinylester resin for standard water de-ionizing use with specific size (diameter 13 inches and height 54 inches), maximum operating pressure 150psi (10.34 bars), maximum operating temperature 150o F (66oC), bed capacity in liters is 105 and the top opening of this vessel is 2½ inches.
The total hardness concentration averaging 528 mg/L was passed through sodium charged strong acid cation exchange resin to reduce the hardness to less than 5 mg/L.
Amberlite IR120Na, also treat with other metal ions like iron and so, the total exchange capacity is become smaller.
The resin was then regenerated using commercially available extra coarse water-softening salt (NaCl). This process was repeated several times to demonstrate that no irreversible fouling had occurred to resin.
Granular activated carbon
Activated carbon is prepared from a char form material such as almond, coconut, and walnut hulls, other woods, and coal.
Activated carbon has the strongest physical adsorption forces or the highest volume of adsorbing porosity of any material known to mankind.
It is a highly porous material; therefore, it has an extremely high surface area for contaminant adsorption
The objective of this topic was to determine the effectiveness of granular activated carbon (GAC) in removing iron from the groundwater.
From these advantages for granular activated carbon, in this case study, we used a single-media filter, .
The depth of the GAC media is estimated based on the average contact time in this media, which is recommended to be 10 to 12 min.
For example, if a filter is designed for a surface loading rate of 4 m3/m2 h, the depth of the GAC media should be at least 0.66 m (4 m3/m2 h ×10 min/60 min per h=0.66 m to 4 m3/m2 h ×12 min/60 min per h=0.8 m, i.e., 0.66 0.8 m)15.
For the following reasons, we used the granular activated carbon in the adsorption of ferrous.
The van der Waals force that forms multilayer adsorption was overcome by the adsorbate due to the high ambient temperature
With relatively high room temperature of about 30oC where the adsorption process occurs, the chemisorption was more dominant as compared to the physisorption.
The relatively high room temperature cause the chemical bond to occurs between the metal ions. Furthermore desorption will also occur between adsorbate and activated carbon at high temperature which physically bonded by the van der Waals force.
Adsorbates which are physically adsorbed onto activated carbon receive sufficient energy from such high temperature to overcome the van der Waals force.
Activated carbon has high adsorption capacity for Fe(II) as compared to others. This may relate to adsorbate characteristics in terms of electronegativity.
The electronegativity of Fe(II) is 1.8 . In fact, electronegativity is a measure of strength for element to attract electron.
In this case, it would measure the strength of Fe(II) attach to negative charge at activated carbon surface.
According to previous literature30, higher electronegativities corresponded to the higher adsorption levels of metal ions onto the GAC.
Another factor that contributes to different GAC adsorption capacity on metal ion is ionic radius.
Fe(II) has relatively smaller ionic radius than that of the others since Fe(II) has the higher attractive charge in nucleus on the electron orbital
The smaller ionic radius of Fe(II) makes it easier to penetrate into the micropores of the GAC.
There were four major functional groups on the surface of activated carbon which are carboxyl, carbonyl, hydroxyl, and lactonized carboxyl
All these four functional groups were promoted to attract cation to it and ion exchange would occur.
Therefore, the Fe(II) which has positive charge would react and attach onto GAC surface’s functional groups with chemically bonded.
However, the actual chemical reaction between the metal ion and functional groups on the activated carbon surface was complex and difficult to understand.
In the case of iron, oxidation is followed by settling and filtration or filtration alone, depending on the concentration of iron in the water.
In the presence of turbidity (and color) and when the Fe(II) concentration is greater than about 5 mg/ l, settling or flotation would be assisted by a coagulant and/or a coagulant aid.
Direct filtration is used when the iron concentration is less than about 5 mg/l .