Hello Muhammad,

Well it depends on which bioavailability you want to explore. if it's human bioaccessibility (like for health risk in the case of soil ingestion for children), you can try the unified BARGE method. It has been validated for different metals, but I actually don't know if it is the case for Cr. The problem remains to have a reference material which has been also validated with this procedure.

https://www.bgs.ac.uk/barge/home.html

Mr.Muhammad ,good question. Bioavailabilty of heavy metals can be determined by soil test methods,sensitive plant test and soil enzyme assay.In the following reference 0.01M CaCl2 method was used for soil analysis,Spinach was  used as test  plant  along with some enzyme assays.

Screening of soil for assessment of toxicity of heavy metals to organisms. by J.K.Saha and N.Panwar  Current Science ,vol.106 No.2 January 25,2014 pages 300-304.

 DTPA methods can also be used as test to assess the content bio -available heavy metals in soils.The following reference can be consulted.

Reference and Media Diagnostic procedure for the Southern Region of the United States.by S.J.Donohue Virginia Agricultural Experimental Station. College of Agriculture and Life Sciences,Virginia Polytechnic Institute &State University ,Blacksburg,VA 24061-0402 Southern Cooperative Series Bulletin no.374,pp.47( 1992).

Analyst. 2006 Jan;131(1):132-40. Epub 2005 Nov 25.

On-line dynamic extraction and automated determination of readily bioavailable hexavalent chromium in solid substrates using micro-sequential injection bead-injection lab-on-valve hyphenated with electrothermal atomic absorption spectrometry.

Long X1, Miró M, Hansen EH.

Polish Journal of Environmental Studies Vol. 14, No 6 (2005), 869-875

Letter to Editor

Determination of Chromium in Soil

of £ódŸ Gardens

B. Jankiewicz, B. Ptaszyñski

Institute of General and Ecological Chemistry, Politechnical University, Lodz, Poland

Received: March 19, 2004

Accepted: April 11, 2005

J. Serb. Chem. Soc. 76 (1) 143–153 (2011) UDC 633.822+546.76+541.183+543.421:615.9

JSCS–4107 Original scientific paper

143

Analysis of the bioavailability of Cr(III) and Cr(VI) based on the

determination of chromium in Mentha piperita by graphite

furnace atomic absorption spectrometry

SVETLANA ĐOGO1*#, SLAVICA RAŽIĆ1#, DRAGAN MANOJLOVIĆ2#

and LATINKA SLAVKOVIĆ3#

Dear Muhammad

See the similar topic on research gate.

https://www.researchgate.net/post/How_to_extract_the_bioavailable_chromium_from_the_soil

 In the link several followers suggested the use of DTPA as extractant for bio-available  chromium.