Abyzhar... You may refer to some papers  as a start point 

http://superfund.ciesin.columbia.edu/sfund_files/documents/events/ES%26T%20Sarkar_SenGupta.pdf

http://weiss.com/pdfs/Nagar%20et%20al%202010.pdf

http://www.bioline.org.br/pdf?st13001

good luck

 Dear Gibran

please follow the articles:

1- Vithanage, M., Chandrajith, R., Bandara, A., Weerasooriya, R., 2006. Mechanism modeling of 

arsenic retention on natural red earth in simulated environmental systems. J. Colloid.

Interface. Sci. 294, 265-272.

2- Dixit, S., Hering, J.G., 2003. Comparison of arsenic (V) and arsenic (III) sorption onto iron

oxide minerals: Implications for arsenic mobility. Environ. Sci Technol. 37, 4182-4189. 

3-  Sarkar, S. Investigations of well-head arsenic removal units in

West Bengal, India. Ph.D. Dissertation, 2006, BESU, Howrah,

India.

4-Manful GA, Verloo M, Spiegeleer FG (1989) Arsenate sorption by

soils in relation to pH and selected anions. Pedologie 39:55–68.

Regards

Gibran, this may be a good idea. It is better to take observations according to your plan and analysis of the resultant  will show the results.

Thank you so much Mr. Virendra, Dr. Naveen, Ms. Parisa and Mr. Mahmood.

GAC is not a proper adsorbent for arsenic removal. There would be no benefit, unless the GAC was impregnated with Fe(OH)3 or another metal oxide.

The mechanism of removal of arsenic is by oxidation/filtration, whereby the following conditions take place:

1. As(III) is oxidized to form As(V). As(III) is soluble in water, while As(V) is insoluble, but the particulate size is still very small and not possible to remove in a media filter without the help of a coagulant. It is possible to remove As(V) in an ultrafiltration or nanofiltration membrane filter. Membranes are sensitive to the presence of residual oxidants however.

2. Either a granular media bed consisting of many metal oxide sites (e.g. ferric hydroxide adsorption contactor), or the presence of metal oxides in solution that co-precipitate and complex with the arsenic. If the water in question already contains Fe (II), it is necessary that the Fe be oxidized as well since ferrous iron is soluble. In this case, there needs to be a minimum of 3 times the amount of iron (as ion) than arsenic in solution to reduce the arsenic to acceptable levels, with frequently more than this ratio required if conditions are not optimal. Simply add more iron by dosing FeCl3 in that case.

3. Optimal conditions tend to be in the pH range of 6.0-7.5. Excess alkalinity exceeding 100 ppm may make it necessary to adjust pH at considerable cost. The resulting water may have too low an LSI though, and it may be desired to raise pH back up to prevent corrosion of the downstream distribution network.

We have also seen evidence of interference with phosphates. If sequestering agents are being used in the water to maintain iron solubility, it may be challenging to oxidize all of the iron, and that iron will be used up by phosphate adsorption, which appears to be preferential to arsenate. It would be prudent to cease phosphate dosing ahead of arsenic treatment and resume it downstream. But since iron removal will be taking place in the filter, it may no longer be necessary to continue dosing the phosphate in the first place.

We have also seen interference with silica, but we are do not fully understand the reason for it. Silica concentration exceeding 30 ppm will reduce efficiency and require greater iron-arsenic ratio (perhaps 10:1 or more) and lower pH. Silica above 70 ppm will prove very challenging for arsenic removal.

Other notes:

1. Pyrolusite (MnO2) is very effective as a granular media. Either native, as-mined mineral or as a coating on another granular substrate. The presence of an oxidant residual will keep the media in an oxidized state and it may reduce iron and arsenic to concentrations below detection limits.

2. Media filters will require backwashing once or twice a day (depending on contaminant levels). In groundwater, it will not be necessary to air scour the bed and a backwash time period of 5-7 minutes should be all that is required in most cases.

3. The backwash water is sometimes sent to a sanitary sewer, but in large systems, the volume of water could be too great for a WWTP or there may not be a WWTP nearby. The volume of water can be significantly reduced by the use of a decant tank to settle the iron particulate (settle for 4-5 hours), allowing a trickle-return of the decanted backwash water ahead of the filter. This trickle flow tends to not exceed 5% of the filter treatment flow rate. There will be a concentrated ferric sludge in the bottom of the decant tank that will contain levels of arsenic that may approach limits of concern, requiring disposal in a sanitary landfill or further treatment of the sludge to contain arsenic and prevent leaching in a non-sanitary landfill.

4. Sodium hypochlorite is frequently used as an oxidant, but potassium permanganate can also be used and may be desired if granular media is greensand or silica sand, with a manganate coating. The permanganate can be used to "treat" new sand media by soaking in permanganate solution, where the sand will get a manganate coating that can aid in the removal of iron/arsenic. Residual permanganate in the water will have a pink shade to it and is considered undesirable but not harmful. Other oxidants can be considered: ozone, ClO2, etc.

5. Greensand is a delicate structure and cannot be subjected to high throughput or vigorous backwashing, but it is lightweight and can be fluidized easily compared to silica sand or pyrolusite. Pure pyroslusite is very durable but is nearly as dense as garnet so requires very high backwash rate for proper fluidization. Other materials are available on the market which mitigate these effects, but they will usually contain pyrolusite in some form. Ironically, if silica is too low/absent in groundwater, greensand or other delicate media containing silica can be a problem as silica can be leached from the granule structure and cause premature failure (

 Thank you David Salyer. So, do you think that combining AC filter with sand filter can lower the waste product (used sand and used AC)?

Because my theory is that with these combined, the amount of AC used can be reduced, since the residue from that filter will be taken by sand filter, moreover it may be possible to use less volume of sand. It is also known that sand filter can be used for 1.5 week before replacement, and AC maybe 3-4 days before replacement (due to the rapid decrease in surface area of AC)

I really need to know if this hypothesis is worth to research on

Thank you

GAC is used for adsorption of organic compounds. It is only in this sense that the surface area of the GAC is a useful term and is meaningless when referring to arsenic removal (unless it has been altered by impregnation). Again, GAC will have no benefit for arsenic removal. What organic compounds are you trying to remove? Sand is used for filtration of suspended particles. GAC, as a granular media, is capable of filtering particles but it is a poor choice for this. If you are trying to remove arsenic, as I described, it is precipitated and removed as a particle. There is no need to replace the media, just backwash it. I am confused by your question regarding media replacement. The sand media should last many years. If you are trying to adsorb organic compounds, GAC life will depend on the species of organics and their concentrations. GAC suppliers can give a projection if given a water quality analysis.