Arsenic Speciation in Rice and the Environment, 2019


Sanjai J. Parikh, proffessor, Dept. of Land, Air and Water Resources, UC Davis

The goal of this continuing project is to study how arsenic moves through rice systems and how management practices can be used to reduce arsenic uptake during cultivation. The objectives in 2019 were:

  • Examine the binding mechanisms of arsenic to rice root plaques to better understand arsenic bioavailability under alternate wetting and drying (AWD).
  • Characterize paddy soil samples and root plaque from previous single dry-down experiments for their mineral composition.
  • Explore the use of rice straw biomass, rich in silica, as a means to reduce arsenic uptake.
  • To accomplish the first objective, researchers analyzed minerals present in iron plaque in rice roots with infrared spectroscopy. Iron minerals present in iron plaque were synthesized. Ongoing binding experiments between these iron minerals and arsenic are continuing to better understand the interactions and uptake of arsenic in the field.

    For the second objective, iron plaque was extracted from dry root samples at harvest and analyzed under X-ray diffraction to identify the minerals present under different AWD treatments. Minerals that may bind easily to arsenic were found in the highest severity AWD treatment (compared to the continuously flooded treatment), proving that AWD has an impact on the mineralogy of iron plaque that can affect arsenic uptake into grain.

    Finally, researchers evaluated the possibility of using magnetic biochar to remove metals from the soil. Biochar is produced through the thermal conversion of waste biomass, such as rice straw, in conditions of limited oxygen. Researchers mixed rice straw biochar and cedar wood chips with iron oxides to create magnetic biochar, a promising technology for removing contaminants such as metals and pesticides from rice paddies.

    An update from results of the 2018 harvest: AWD reduces arsenic accumulation in shoots and grains but can lead to increased cadmium uptake. Grain samples from 2015 to 2018 were analyzed. Also, AWD does not negatively impact nutritional values for phosphorous, iron, potassium, and zinc and actually increased them slightly, confirming that it is possible to find an optimal AWD treatment that can reduce contaminants while increasing nutrient concentrations in grain.