Assessing equilibria of organo-arsenic complexes and predicting uptake of arsenic by wheat grain from organic matter amended soils.

In view of limited information, a laboratory experiment was conducted to study the stability of organo-arsenic complexes as affected by competing anions i.e. phosphate, nitrate and sulphate. For this purpose, humic acid (HA) and fulvic acid (FA) were extracted from farmyard manure (FYM), vermicompost (VC), sugarcane bagasse (SB) and soil. A pot experiment was also conducted with 4 levels each of arsenic (As) (10, 20, 30 and 40  mg  kg) and amendments (no amendment, FYM, VC and SB at the rate of 10  t  ha each). Results indicate that stability of FA extracted from sugarcane bagasse have the highest stability constant (log K) as 9.77 and the corresponding mole ratio (x) value of 1.51. The phosphate was the most effective in replacing As from organo-As complexes followed by sulphate and nitrate. Under pot culture study, As content in wheat grain was the lowest in sugarcane bagasse amended soil followed by FYM and VC at all levels of As application. Solubility-free ion activity model was most effective in predicting As uptake by wheat grain based on Olsen extractable As, pH and Walkley & Black organic C. Efficacy of organic amendments in reducing health hazard for intake of As through consumption of wheat grain grown on contaminated soil was also reflected in the values of hazard quotient (HQ).