Electrocoagulation with Iron-based electrodes is reliable and cheap to reduce CrIV to CrIII.

I suggest to consult the following papers containing useful information on chromium (and hexavalent chromium) removal from wastewater and sewage sludge: A. M. Ferro Orozco et al., Journal of Hazardous Materials 150 (2008) 46–52 (Modelling Cr(VI) removal by a combined carbon-activated sludge system); J. Ye et al., Bioresource Technology 101 (2010) 3893–3902 (Biosorption of chromium from aqueous solution and electroplating wastewater using mixture of Candida lipolytica and dewatered sewage sludge); L.H. Zhang and Z.L.Zhu, Pedosphere 22(1) (2012) 131–136 (Chromium Extraction from Sewage Sludge Using Polyepoxysuccinic Acid).

Try oxidizing agents

reducing agent like elemental iron will definately work

yeah you can try one of my methods: just take the solution of whatever concentration of Cr(VI) about 20 ml and add around 5 ml of Glycerol. Stir the solution properly and add 5 ml of Conc H2SO4 . After 15 minutes the colour changes from Orange to Green. Remember Green colour is the indicator of reduction produced by Cr(III).

Industrial applications are still to perform..........

Thank you for your time and answers they are greatly appreciated.

I would suggest an electrochemical method which would not deliver additional chemicals into the environment; being not a specialist in water treatment engineering but an electrochemist I would use a stainless steel net and set its potential to the one needed for the reduction. Just verifying my first thoughts I found a corresponding patent: EP 1957412 A1

the residual water from Cr (VI) deposition is reduced using NaOH. the precipitate is Cr (OH)3.

Sulfurous acid can also be used (at pH ~5.0) to promote oxidation of S(IV) to S(V) and/or S(VI) by Cr(VI), which reduces to Cr(III).

At pCr > 1.5 the following parallel reactions can be considered:

2 HCrO4- + 4 HSO3- + 6 H+ → 2 Cr3+ + 2 SO42- + S2O62- + 6 H2O

2 HCrO4- + 3 HSO3- + 5 H+ → 2 Cr3+ + 3 SO42- + 5 H2O

At pCr < 1.5 the following reactions appear as the most plausible, because Cr2O72- would then predominate over HCrO4-, while in the former case the opposite occurs:

Cr2O72- + 4 HSO3- + 6 H+ → 2 Cr3+ + 2 SO42- + S2O62- + 5 H2O

Cr2O72- + 3 HSO3- + 5 H+ → 2 Cr3+ + 3 SO42- + 4 H2O

Cf. D. Benson, "Introduction aux Mécanismes des Réactions Inorganiques en Solution", translated from "Mechanisms of Inorganic Reactions in Solution: An Introduction" (McGraw-Hill, London, 1968), Masson et Cie, Paris, 1972, p. 146;

Predominance diagram, http://en.wikipedia.org/wiki/Predominance_diagram.

The most easy way is to use Fe(II) as reducing agent. Fe(II) reduces Cr(VI) to cr(III) and Fe(II) goes to Fe(III). At ph over 7, Fe(III) and Cr(III) can co-precipitate.Another way is to use sulfide..There are lots of paper. If you need help, please let me know.

Sulphate reducing bacteria can also be a solution cause basically we are dealing with oxidation of Cr VI to Cr III...........

this can be used........

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2849200/

First deceas the pH below 3 pH unit , Then add a reduction agent sach as SO2, bisolphatew etc , last increas the pH abov 7.5 pH unit, the later is possible by adding hydrated lime

Besides the inorganic matter such as sulfide compounds and metallic iron, you can use an organic material such as polysaccarides to reduce Cr(VI) to Cr(III) under the mildly acidic conditions.

Chromate species + Organic reductants + Acid --> Cr(III) + H2O+CO2 (and/or oxidised product)

Ozer, A., Altundogan, H.S., Erdem, M., Tumen, F., A Study on The Cr(VI) Removal from Aqueous Solutions by Steel Wool, Environmental Pollution, 97, 107-112, 1997.

Kiyak, B., Ozer, A., Altundogan, H. S., Erdem, M., Tumen, F., Cr(VI) Reduction in Aqueos Solution by Using Copper Smelter Slag, Waste Mamagement, 19, 333-338, 1999.

Erdem, M., Altundogan, H. S., Ozer, A., Tumen, F., Cr(VI) Reduction in Aqueous Solutions by Using Synthetic Iron Sulphide, Environmental Technology, 22, 1213-1222, 2001.

Altundogan, H.S., Cr(VI) Removal From Aqueous Solution by Iron (III) Hydroxide-Loaded Sugar Beet Pulp, Process Biochemistry, 40, 1443-1452, 2005.

Even Chloride (CL-) under many conditions will be oxydized to chlorine elemental gas by the reduction of chrome six to chrome III!. That is why spontaneous oxidation of chrome III into chrome six is very unlikely since chloride is so ubiquitous as most of our planet has ordinary salt present in our oceans! The problem consists in predicting in terms of pE where can chromium plus three turn into chromium plus six! The Pourbaix corrosion of metals vs pH diagrams are a good way to predict under what conditions is this possible or not!

Many solutions, some easy some little difficult:

Standard ways: SO2, thiosulfate and Fe++

Alternative ways: organic reducing agents

Undisclosed ways: H2O2 + CrO4- form at pH under 5 the perchromates (blue) that are unstable and evolve to Cr+++ and O2. O2 requires some time to be developed so the reaction can seem slow

Cr III is not only not particularly toxic to mamalian metaboliisms , but is actually a rerquired trace element for life to exist!  Chrome III is present in the biosphere and is not dangerous to living creatures.  Chromium can coexist with humsans!

My question is the opposite. is there any suggestions to oxidize natural chromium (+3) to +6? 

Thanks all

hI, mISS RETTA:

The problem is, not only the reduction from Cr(VI) to (III), but to fix Cr at this oxidation state. In case of solid waste Cr should be transformed in spinel, that is a stable form. 

Electrocoagulation with Iron-based electrodes is reliable and cheap to reduce CrIV to CrIII.