Aomi Suda

The effects of soil amendments on arsenic concentrations in soil solutions after long-term flooded incubation

Abstract We investigated the effects of various soil amendments on the concentration of arsenic (As) in soil solutions under flooded anaerobic conditions. Ten amendments (six As adsorbents and four general agricultural amendments) were used with two soil samples (Aquepts). The time-course changes in the concentrations of As, iron (Fe) and silicon (Si) in soil solutions with or without amendments were measured after incubations of approximately 30, 60 and 100 d. We conclude that a precipitate of polysilicate-iron solution (1.1 mol L‒1 Fe) shows promise as a novel As adsorbent for flooded anaerobic soils such as paddy soils. The precipitate is present in sludge from water purification plants and is therefore potentially cost effective. Moreover, it could increase the concentration of Si in soil solution, which could decrease As uptake by rice plants. Ferrihydrite and aluminum-substituted ferrihydrite were also effective for decreasing the dissolved As in both soils and nano-sized layered double hydroxides, and magnesium oxides worked in one soil. Poultry manure significantly increased dissolved As in both soils, but composted bark and silicate fertilizers did not change As in either soil. Some amendments had different effects in the two soil samples; further comprehensive studies will need to focus on how soil properties influence the effects of amendments.

Use of water-treatment residue containing polysilicate-iron to stabilize arsenic in flooded soils and attenuate arsenic uptake by rice (Oryza sativa L.) plants

ABSTRACT A pot experiment was conducted to examine how soil amendment with water-treatment residue (WTR) containing polysilicate-iron affected dissolved arsenic (As) in flooded soils and As uptake by rice plants (Oryza sativa L.). The WTR was applied at a rate of 0 (control), 5, 10 or 20 t ha−1. Simple linear regression analyses showed significant negative relationships between the concentrations of dissolved As in soil solution and WTR application rates at all sampling times, probably due to adsorption of As onto ferrihydrite in the WTR. Compared to As concentrations in rice plants grown on control soil, the concentrations in plants grown on WTR-treated soils decreased by 20.1–41.6% in straw (stems and leaves), 19.8–31.7% in husk and 18.6–21.0% in grain. The regression analyses demonstrated that the concentration and content of As in rice are negatively correlated with WTR application rate. Total As content was 16.5–32.0% lower in rice shoots grown on WTR-treated soils than on control soil. The percentage of each As species in grain decreased in the following order: As(III) » dimethylarsinic acid » As(V). The application of WTR did not change the As speciation in grain. Silicon contents in shoot significantly increased with application of WTR, indicating the potency of WTR as a silicate fertilizer. Taken together, our results indicate that WTR containing polysilicate-iron promises to be a practical amendment for stabilizing As and attenuating As uptake by rice plants.

Simultaneous decrease of arsenic and cadmium in rice (Oryza sativa L.) plants cultivated under submerged field conditions by the application of iron-bearing materials

ABSTRACT 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.

Effects of sterilization on the chemical forms of heavy metals in soils.

Abstract We used sequential extraction to investigate changes in the amounts of six chemical forms of manganese, cobalt and cadmium in soil samples after chloroform fumigation. The six forms were designated as follows: exchangeable, dilute-acid-soluble, manganese-oxide-occluded, organically bound, iron-oxide-occluded and residual. For all three metals, the decreases in the amounts of manganese-oxide-occluded forms were equivalent to the sum of the increases in the amounts of exchangeable and dilute-acid-soluble forms. The amounts of the other three forms did not change significantly after fumigation. These results indicate that some of the cobalt and cadmium in the manganese oxides was converted into exchangeable and dilute-acid-soluble forms, which suggests that soil sterilization may increase the availability of these heavy metals to plants.

An improved selective extraction method for Mn oxides and occluded metals with emphasis on applicability to Andisols

Abstract It has been showed that Chao’s method [extraction with 0.1 mol L−1 hydroxylamine hydrochloride (NH2OH-HCl) at pH 2.0 for 30 min], which is commonly used to extract manganese (Mn) oxides and occluded heavy metals from soil samples, is not suitable for Andisols because of low solubility, and thus low extractability, of Mn oxides in such soils. Therefore, a new method is evaluated here, for extracting Mn oxides and occluded heavy metals from Andisols, Entisols and Inceptisols. The method has three steps: (1) reduction of Mn oxides with 0.01 mol L−1 NH2OH-HCl (pH 5.0) for 16 h, (2) recovery of re-adsorbed metals by short-time extraction with 0.5 mol L−1 ammonium chloride in 0.02 mol L‒1 hydrochloric acid, and (3) washing with ultrapure water. This method achieves a higher rate of extraction of Mn oxides than does Chao’s method, especially from Andisol samples. Standard addition experiments showed that both the new method and Chao’s method can successfully extract released cadmium (Cd), cobalt (Co), nickel (Ni) and zinc (Zn) from Mn oxides with little re-adsorption. The selectivity of Mn oxide extraction by the new method, indicated by the rate of extraction of iron (Fe) oxides and the aluminum (Al)/Mn and silicon (Si)/Mn extraction ratios, is comparable to that of Chao’s method. Thus, the new method should be useful for extracting Mn oxides and occluded Cd, Co, Ni, and Zn from soil samples. Moreover, because the new method achieved nearly complete extraction of NH2OH-HCl reactive Mn oxides even from Andisol samples, the method is more applicable to Andisol samples than Chao’s method.

Improvement of the NH2OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan

Abstract We evaluated the validity of Tessier’s method as applied to the extraction of manganese (Mn) and iron (Fe) oxides in Japanese Andisols and other soil types in Japan. Using the original Tessier’s extractant mixture, 0.04 mol L−1 hydroxylamine hydrochloride in 25% acetic acid (0.04 mol L–1 NH2OH-HCl in 25% HOAc), we found that substantial amounts of short-range-ordered Fe oxides were not extracted from allophanic Andisol samples and that considerable amounts of total Fe oxides were not extracted from all soil types. Relatively high extraction pH and large amounts of short-range-ordered Fe oxides in the Andisol samples might be responsible for incomplete extraction. Stoichiometric calculation indicated that the concentration of NH2OH-HCl might be insufficient for complete extraction of Fe oxides. The extracted amounts of Mn and Fe increased with increasing concentration of NH2OH-HCl in the extractant, and most of the Mn and Fe oxides in the soil samples, including samples with as much as 5.6% Fe, were extracted with 0.6 mol L–1 NH2OH–HCl in 25% HOAc. As judged from the simultaneous dissolution of aluminum (Al) and silicon (Si) minerals, extraction selectivity of Fe oxides with 0.6 mol L–1 NH2OH-HCl in 25% HOAc was comparable to that of the original Tessier’s method and better than that of a modified Community Bureau of Reference (BCR) sequential extraction procedure or a method using an extractant consisting of a mixture of oxalate and ascorbate, especially for Andisol samples.

Extractability of manganese and iron oxides in typical Japanese soils by 0.5 mol L−1 hydroxylamine hydrochloride (pH 1.5)

We investigated the extractability of manganese (Mn) and iron (Fe) oxides from typical Japanese soils (Entisols, Inceptisols, and Andisols) by 0.5 mol L−1 hydroxylamine hydrochloride (NH2OH-HCl) extraction (pH 1.5; 16 h shaking at 25°C; soil:solution ratio 1:40), referred as to HHmBCR, which is Step 2 (used for the reducible fraction) of the modified BCR (Community Bureau of Reference) sequential extraction procedure. The HHmBCR procedure extracted almost all Mn oxides from the non-Andisol samples, but failed to extract a part of the Mn oxides from some Andisol samples. The procedure extracted most short-range ordered Fe oxides from non-Andisol samples, but it extracted only 7.5% and 13% of the short-range ordered Fe oxides from allophanic and non-allophanic Andisol samples, respectively. This remarkably low extractability of Fe oxides suggests that the HHmBCR method is not suitable for extracting oxide-occluded heavy metals from Andisols. Since the extraction rate of short-range ordered Fe oxides from various soils with the extractant was negatively correlated with the amounts of oxalate- and pyrophosphate-extractable Al even when the variability of the extraction pH was reduced by increasing the soil:solution ratio from 1:40 to 1:500, the extractability of Fe oxides would be negatively affected by the presence of active Al, including allophane/imogolite, amorphous Al, and Al-humus complexes. Because these Al constituents are abundant in Andisols, they would be at least partially responsible for the lower extractability of Fe oxides by HHmBCR from Andisols.

Arsenic immobilization in anaerobic soils by the application of by-product iron materials obtained from the casting industry

ABSTRACT Reducing the arsenic (As) concentration in rice grains is of great interest from a human health perspective. Iron (Fe) materials immobilize As in soils, thereby effectively reducing the As concentration in rice grains. We investigated the effect of by-product Fe materials obtained from the casting industry on the As mobility in two soils (soil A and soil B) by a long-term (approximately 100 days) flooded soil incubation experiment. The examined Fe materials were spent steel shot (SSS), fine spent casting sand (SCS) containing steel shot, and two kinds of residual Fe materials (RIMs) from steel shot production. Commercial Fe materials used to immobilize As (zero-valent Fe and ferrihydrite) were tested for comparison. The dissolved As in soil solution of controls for soil A and soil B reached approximately 100 and 800 μg L‒1, respectively. The effect on As immobilization of all the by-product Fe materials increased with time and was comparable to or greater than that of commercial ferrihydrite, except for SCS. The additions of SSS and RIMs decreased by more than 90% of the dissolved As in soil A and decreased by more than 50% in soil B after 100 days incubation. Overall, the effect of the by-product Fe materials on the solubility of silicon and phosphorus was much less than that of the commercial Fe materials. Considering the cost advantage over commercial Fe materials, the Fe materials obtained from the casting industry as by-products are promising amendments for the immobilization of As in paddy soils.

Attenuation of inorganic arsenic and cadmium in rice grains using by-product iron materials from the casting industry combined with different water management practices

ABSTRACT We examined the effect of two types of iron (Fe) material produced by the casting industry (spent steel shot [SSS] and residual iron material from steel shot production) on the mobility of arsenic (As) and cadmium (Cd) in soils. We also examined the uptake of these elements by rice plants (Oryza Sativa L.) under continuously flooded (CF) and water-saving (WS) cultivation. The application of both Fe materials (at 10 and 30 t ha‒1) strictly limited As mobilization in soils under CF cultivation. As a result, As uptake by rice plants declined, along with the total and inorganic As (iAs) concentration in rice grains. In comparison, As immobilization caused by the application of Fe material was less clear under WS cultivation. The rate of Fe material application was negatively correlated with As uptake by rice plants. It was also negatively correlated with total and iAs concentration in rice grains under both water management practices. The combination of applying Fe materials and WS cultivation decreased iAs concentration in rice grains to approximately one-fifth of that in rice grains produced from plants grown on soils without Fe material application under CF cultivation. CF cultivation strictly decreased dissolved Cd in soils, as well as Cd in rice grains with and without Fe material application. The application of Fe materials decreased Cd mobility and, hence, Cd uptake in rice plants, ultimately reducing the accumulation of Cd in rice grains under WS cultivation. Residual Fe material had a statistically greater effect at attenuating Cd accumulation in rice grains than SSS. The present study demonstrated the potential of combining by-product Fe material application and water management practices to attenuate iAs and/or Cd concentrations in rice grains. Practical countermeasures should be carefully adopted that consider the existing risks of iAs and Cd on each paddy field, and the combined effect of Fe material application and water management practices.