Phosphorus fertilizers from less pure sedimentary sources become increasingly important, due to depletion of phosphorus from igneous rock of high quality.Consequently, robust methods with potential to remove various types of hazardous elements are required.Among such impurities, hexavalent chromium (Cr(VI)) is very likely to become a future challenge.Different industrial ways to treat phosphate rock are currently being practised, and we have here studied how chromium behaves when using the nitro-phosphate process.The reduction mechanism of Cr (VI) in nitric acid and phosphoric acid solutions was investigated by measuring redox potential and UV-VIS spectra.The results show that Cr (VI) is not stable in strong nitric acid solutions.Reduction of Cr (VI) species decreased with decreasing temperature, NO 2 concentration, ionic strength and absence of light.These findings support the proposed reduction reaction: ( ) ( ) 2 2 7 2 Cr O aq 6NO 2H aq -+ + + → ( ) ( ) 3 2 3 2Cr aq H O 6NO aq .+ -2 1 3 2 3 2 Rate Cr III k HNO Cr VI k k H O ⋅ ⋅ + ⋅ ∝ .
