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I,  humbly, will discuss 2-3 methods each for Cr and [As].

[A] For Cr

{I}The method/ experimental details are noted from two references given at the end for your perusal and authentication.

[A]It can be removed by Electrokinetic remediation[applied in situ] by applying 50-150V depending upon the nature of soil having moisture[ but not having saturated condition], low salinity, low conduction.

[B]It is applied in situ, with electrodes on both sides of the contaminated soi/slurry with the electrode placed directly on the ground to the depth of 3-5 meters.

[C]The Cr(VI) cotaminants will concentrate around the anode in aquous phase. The anode reservoir cotained a column packed adsorbent (immobilized tannin* chemically incorporated into cellulose). Cr(VI) gets adsorbed to this adsorbent .

[D] The pH of the aqueous solution in the anodic reservoir is decreased by the electrolysis of water but is kept around 6 ( by the proper addition of an aqueous alkaline solution ).

[E]The advantage of pH control is that it promotes the release of Cr(VI) from the soil by electromigration, thus permitting the maximum adsorption of Cr(VI) on the immobilized tannin. Simultaneous collection of Cr(VI) from the anodic reservoir leads to the protection from secondary contamination with Cr(VI).

TWO REFERENCES.

[A] Chromium(VI) Handbook[page-296]

edited by Jacques Guertin, James A. Jacobs, Cynthia P. Avakian

[B]Waste Management

Volume 24, Issue 5, 2004, Pages 483–490.

*Rather than causing any injurious / harmful effects to the soil/ plants, tanin plays a role in protection from predation and also as a pesticide which is helpful in plant growth.

[III] In addition, two very useful characteristics of tannin are:

(a)It is highly leachable water soluble compound.

(b)What makes it all the more useful is that raising of pH accelerates the process of leaching. And your self would have noted in the experimental details of my humble answer that we always raise the pH by adding an alkaline solution.

Method-2

By the repeted use of FeSO4.

Method -3

Repeatedly growing BIOMASS crops or trees - Bamboo/ Cauarina.

[B] For As

Method-I.

Fe (+2, 3), Mn (+2) and Al (+3) oxides are cosidered to be principle solid phase components which control  adsorption of arsenic in soils. Most of these components which were insoluble become soluble if there is excess of nitrate in the soil and may extrete by watering the soil or on raining .As at the maximum 51% of [As} can be extracted in one trial, so the excess of NaNO3 should also be avoided, and  quantities of NaNO3 to be added are adjusted  depending upon  % of As present in the soil sample as follows:

Take 0.5g of the given soil sample .Add 25.0 ml of 0.1M NaNO3.Shake for 24 hrs. Centrifuge at 4000 rpm for 5 minutes. Transfer the supernant liquid in a 60 .0 ml plastic bottle.Add 23.0 ml of DDI water and 2.5 ml of HNO3 and discard the residue soil.The arsenic present in the soil sample is oxidized to sodium arsenate.Transfer 10.0 ml from the plastic bottle in a test tube.Add 50.0 micro litres of internal standard.All the chemicals should be of AR grade. Perfom standard inductively coupled plasma-mass spectrometry (ICP-MS) analysis. Repeat the experiment at least three times. Calculate the amount of arsenic extracted by NaNO3 and compare it with the originally known concentration of the soil sample.  

Method-2.

Arsenic can be removed in groundwater by a method called Phytoremediation which consists of passing contaminated water through the specially constructed wet land bed with Hyperaccumulator plants such as P.cretica , Pteris vittata, P. longfolia, P. unbrosa (fern type), which can take up up to 1% [As].

Method-3

Use of phosphate fertilizers will be helpful in[ As ]removal capacity of the crops.

Method-4.

Since, [As] concentration is found up to 20 -22g/kg in marshy places, grow any plant in the land containing (As)which grows in such marshy places .

Dear Prof Manohar Sehgal,

Thank you very much for your kindness and time to give the background knowledge about Cr and As contents in soil. I will appreciate if more details on the real situation about As and Cr contaminated water issues in India and the efforts which are being actually made by some technology to provide the safe drinking water to the people. For example, How much population is facing the danger of As and Cr exposure through contaminated water, say in India, and which parts of India especially need early or immidiate steps in this direction, if any? Many thanks and best regards,

Hi Dr Brahm Pal Singh ji; I am sending few links which could be useful to you.

http://www.sciencedirect.com/science/article/pii/S0045653504007684

http://journals.lww.com/epidem/Abstract/2003/03000/Arsenic_in_Drinking_Water_and_Skin_Lesions_.11.aspx

http://www.sciencedirect.com/science/article/pii/S0883292702000185

Regards

Dear Dr Sarwan Kumar Dubey ji, Thanks a lot for references giving details of the As related/effected regions. Very helpful to understand real situation of As contaminated underground drinking water in WB, India. It will be great to know whether there are any measures or not to address such health related serious issues for such a large population in West Bengal? I am very much interested to learn/know more of any such actual measures being taken or not in India and/or else where?

Best regards,

Dear Brahm

As I know and teach in my courses on environmental geochemistry, the international organizations have been involved upon the media announced the disaster in WB and Bangladesh and they found that 60 million people are subjected in areas. I have not heard about measures taken. I frequently read that the local people invented some filters by using iron coated sand to remove As from drinking water. No important measure  has been taken in practice (Based on my knowledge) however you may find myriads of researches on this area just about distribution and health impact. This is also the case in my country. We can research on sources of arsenic and publishing many papers on how to remove arsenic by using nanoparticles, iron, zeolites, organic absorbents etc. None of them came into practice.

regarding Arsenic:

http://projectwellusa.org/index.php

http://www.sciencedirect.com/science/article/pii/S0269749109004643

http://www.worstpolluted.org/projects_reports/display/76

Chromium:

http://www.blacksmithinstitute.org/success_stories/display/28

Ethical war against pollution will never end by giving scientific solutions to pollution generated, but need to stop pollution sources by legal ways except for metals from natural sources (e.g. Arsenic).

Dear Profs Soroush Modabberi and Arvind Behal,

Thank you very much for your useful information and related URLs for As and Cr.

However, I wonder that

1. How sand is coated with iron? and 

2. What mechnism is responsible for removing As without using any light exposure?

By the way, it is often considered that As(III) and Cr(VI) can be converted to their following less poisonous forms As(V) and Cr(III) using several photo catalytic reactors. Does it mean that converted As(V) and Cr(III) can be consumed assuming that the water became safe for drinking ?

If yes, what are their respective permissible concentrations for both new forms of these hazardous metals?

If not, how these new forms of As(V) and Cr(III) can be removed for the safe drinking water?

I will highly appreciate if some one knows the same and can explain it to me.

Thanks a lot in anticipation.