Arsenic (As) and cadmium (Cd) concentrations in rice grains are a human health concern. We conducted field experiments to investigate optimal conditions of Eh and pH in soil for simultaneously decreasing As and Cd accumulation in rice. Water managements in the experiments, which included continuous flooding and intermittent irrigation with different intervals after midseason drainage, exerted striking effects on the dissolved As and Cd concentrations in soil through changes in Eh, pH, and dissolved Fe(II) concentrations in the soil. Intermittent irrigation with three-day flooding and five-day drainage was found to be effective for simultaneously decreasing the accumulation of As and Cd in grain. The grain As and Cd concentrations were, respectively, linearly related to the average dissolved As and Cd concentrations during the 3 weeks after heading. We propose a new indicator for expressing the degree to which a decrease in the dissolved As or Cd concentration is compromised by the increase in the other. For minimizing the trade-off relationship between As and Cd in rice grains in the field investigated, water management strategies should target the realization of optimal soil Eh of -73 mV and pH of 6.2 during the 3 weeks after heading.

Andisols containing allophane and other poorly crystalline materials, carry variable surface charge, and adsorb otherwise inert anions such as Cl− and NO−3. The objective of this study was to predict composition‐dependent, retarded transport of anions during unsteady water flow in an Andisol. One‐dimensional absorption experiments in Kannondai subsoil (Hydric Hapludand) columns were conducted with CaCl2, Ca(NO3)2, and mixed CaCl2‐Ca(NO3)2 solutions at different concentrations (0.00462‐0.115 mol L−1). Because of nonlinear, competitive adsorption of Cl− and NO−3 by the soil, the observed retardation of anion penetration depended on the total concentration and composition of the invading solution. Nitrate ion had a smaller affinity to the adsorption sites than Cl− (the selectivity coefficient Kv = 0.51) and showed a greater penetration in the presence of Cl−. The adsorption of Cl− and NO−3 much exceeded the concurrent SO2−4 desorption from the soil. This suggests that SO2−4 was so strongly adsorbed that exchange reaction with the invading anions proceeded only to a limited extent. Thus, the adsorption of the monovalent anions was largely due to the increase in the anion exchange capacity of the soil in response to an increase in the ionic strength of the bulk solution. An approximate theory was developed for anion transport involving competitive adsorption in which total monovalent anion adsorption, obeying a Langmuir‐type equation, is allocated via Kv to the adsorption of Cl− and NO−3. Employing the adsorption parameters determined from separate steady state leaching experiments, the theory successfully described the measured anion content profiles.

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The amount of reactive ions adsorbed during transport in variable‐charge soils is often smaller than those predicted from batch adsorption experiments. This overestimation of adsorption for invading ions by the batch method is likely to be caused by excessive desorption of indigenous, strongly adsorbed ions. We propose an unsaturated transient flow method by which equilibrium adsorption of weakly reactive ions can be determined without causing appreciable desorption of indigenous ions. In our method, water is imbibed into horizontal columns packed with soil that has been mixed with a salt solution at different concentrations to attain adsorption equilibrium. We make use of the observation that during imbibition of water, the antecedent solution is pushed ahead by the invading solution and accumulates in the region beyond the plane of separation, with the solution concentration unchanged. From linear plots of the solute content vs. soil water content in this region, the amount of solutes adsorbed by soil and the equilibrium solution concentration prior to the water imbibition are obtained. The advantages of the method are that adsorption isotherms can be determined under conditions similar to those expected during transport processes in soil, and that it is exempt from uncertainty about attainment of adsorption equilibrium as in the miscible displacement methods. The proposed method is best suited for variable‐charge soils where adsorption of weakly reactive ions is largely due to an increase in the exchange capacity.

The Codex Alimentarius Commission has recently adopted maximum levels for inorganic arsenic (As; in 2014) and total cadmium (Cd; in 2006) in polished rice grains to maintain food safety and to decrease the risk to human health. As rice is a staple crop in Japan and monsoon Asian countries, reducing concentrations of As and Cd in rice is an urgent matter. In flooded conditions, Cd concentration in soil solution decreases whereas As concentration increases. Therefore, we aimed to evaluate the efficiency of iron-bearing materials to decrease As concentration in soil solution and rice (Oryza sativa L.) grain under submerged cultivation, while also considering Cd concentration. In experiments conducted in paddy fields in six regions, As concentrations in the soil solution during the cultivation period decreased in the following order: control (REF) > steel converter furnace slag (SCS) > non-crystalline iron hydroxide (FH) > zero-valent iron (ZVI). The concentrations of As in brown rice were in the same order, with ZVI achieving particularly strong reduction. Cadmium concentrations were low, probably owing to submerged cultivation conditions. Application of iron-bearing materials slightly and insignificantly reduced the yields of brown rice and straw. Application of these materials did not have a significant negative impact on the quality of rice. Our data indicate that the application of iron-bearing materials effectively reduces As concentrations in soil solution and rice grains without negative effects on yield and quality, with a particularly powerful effect of ZVI which is possibly explained by arsenic sulfide formation.