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Current mitigation techniques for Arsenic and Cadmium contaminated paddy soils in Korea

Won-Il Kim , Anitha Kunhikrishnan

1Chemical Safety Division, Department of Agro-Food Safety, National Academy of Agricultural Science, Wanju-gun, Jeollabuk-do 565-851, Republic of Korea

*Corresponding author’s email: [email protected] (WI Kim)

Abstract

Managing low concentrations of arsenic (As) and cadmium (Cd) together in rice (Oryza sativa L.) plants is

challenging and recently different strategies are being developed for mitigating As and Cd loading into the rice

grains. Water management affects the bioavailability of As and Cd in the soil and hence their

accumulation in rice grains and grain yields. This study investigated the effect of water management and

the use of soil amendments on As and Cd accumulation in brown rice. We also conducted rice washing experiment to

study the effect of washing by deionized water on the removal rate of inorganic As in polished rice. A field plot

experiment was conducted with five water management regimes [Flooded, alternate wetting drying (AWD – 60 and

40)) and row (R-60 and R-40)] using two rice cultivars (Indica and Japonica). In another field experiment, the effect

of soil amendments, phosphate, silicate and rice husk biochar on As and Cd accumulation in brown rice was

investigated. All the four treatments significantly reduced the concentration of As in brown rice with R-40 showing

the least concentration with no noticeable differences between the cultivars. However, increasing Cd concentrations

were noticed in row and AWD treatments compared to the flooded treatment. While phosphate reduced As

accumulation in SK cultivar, silicate increased As concentration in both the cultivars. Biochar increased As

concentration in SK cultivar with no significant change in HK cultivar, but the trend was opposite for Cd. Silicate

significantly decreased the Cd accumulation only in SK cultivar. The inorganic As levels in polished rice after three

washings were reduced to 37-57% of those in raw rice samples. Although water management and addition of

amendments like phosphate and silicate offer some promising solutions, additional field studies and other

management strategies are required to control both As and Cd in paddy soils and rice grains.

As and Cd reduced by %

Keywords

Introduction

Heavy metal(loid) contamination in paddy soils is one of the most serious problems facing rice production and

soil management in Asian countries. Among agricultural food products, rice (Oryza sativa L.) is one of the most

widely consumed staple cereal foods in the world constituting about 89% of the diet of people in Asian countries. In

many East and South Asian countries including Bangladesh, Japan, Indonesia, and Korea, the accumulation of

metal(loid)s, particularly arsenic (As) and cadmium (Cd), in rice ecosystems and its subsequent transfer to the human

food chain is a major environmental issue. In Korea, rice is the most common crop grown on agricultural land. The

cultivated land in Korea is around 21%, and of that 61% is paddy fields. Large areas of agricultural land, including

paddy fields, have been contaminated by metal(loid)s including As and Cd via effluent from mine tailings and other

wastes generated by closed or abandoned mines (Kim et al., 2007; Kwon et al., 2013; Yang et al., 2006) and thus

results to the uptake of metal(loid)s by rice plants, posing a significant public health risk to the local community.

Rice contains high concentrations of As relative to other crops and chronic exposure to As has the potential to cause

several forms of cancer and various other serious negative health effects in humans. However, the toxicity of As

depends heavily on its chemical environment, with the inorganic forms of As (As(III) and As(V)) being the most

toxic, while the other organic forms of As [monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)], are

generally considered to be mostly non-toxic. Therefore, the potential toxicity of As depends not only on the total As

concentration, but also on the species of As present in a given food sample [4].

While As is immobilized under oxidizing conditions and solubilized under reducing conditions these trends are

the opposite in Cd. Prolonged submergence to keep paddy soils under reducing conditions has been shown to

successfully decrease Cd concentration in rice grains. However, it leads to an increased uptake of As. Managing low

concentrations of As and Cd together in rice plants has always been a serious issue and there is an urgent need to

develop strategies for reducing As and Cd loading into the rice grains. Numerous studies have reported that water

management affects Cd (Kikuchi et al. 2008) and As (Somenahally et al. 2011; Spanu et al. 2012) bioavailability in

soils and their subsequent uptake by rice. When a paddy field is flooded and the soil has a low redox potential, any

Cd present in the soil combines with sulphur (S) to form CdS which has a low solubility in water (Bingham et al.

1976). Thus, flooding during the growing season, especially during later stages of plant growth, can effectively

reduce Cd concentrations in rice grains (Arao et al. 2009). In contrast, anaerobic conditions in paddy soil lead to the

reduction in arsenate to arsenite which enhances the bioavailability of As to rice plants. Therefore, growing rice

aerobically results in a significant decline in As accumulation in rice (Li et al. 2009). However, both Cd and As can

occur together as contaminants in paddy fields, and they can accumulate simultaneously in rice plants (Williams et

al. 2009). To help reduce water consumption during rice cultivation there has been considerable interest in expanding

the aerobic cultivation practices employed in upland rice to lowland environments where anaerobic, paddy

cultivation is traditional (Bouman et al. 2005). However, the reduced yields and pest control problems associated

with aerobic cultivation need to be addressed (Kreye et al. 2009). One major recent advance in rice water

management is termed alternate wetting and drying (AWD) (Price et al. 2013). AWD combines the beneficial aspects

of both aerobic and anaerobic cultivation. INCLUDE ROW

Compared with water management, the potential effects of soil amendments on As and Cd accumulation

have also been studied. Our results demonstrate that applying Si to the growth medium markedly decreased As

concentrations in the shoots and roots and total As uptake by rice seedlings (Guo et al 2005; 2007). The use of

biochar to reduce the mobility and bioavailability of heavy metal(loid)s in contaminated soils has attracted much

attention in recent years. However, the effect of biochar addition on the mobility of As was inconsistent among

different laboratory studies (Beesley and Marmiroli, 2011; Beesley et al., 2011). Studies showed that washing rice

before cooking reduced the total/inorganic As compared to raw rice (Mihucz et al., 2007; 2010).

With all these points taken into consideration, our study investigated the effect of water management and

the use of soil amendments (phosphate, silicate and rice husk biochar) on As and Cd accumulation in brown rice in

two rice cultivars grown in Korea. A rice washing experiment was also conducted to study the effect of washing by

deionized water on the removal rate of inorganic As in polished rice. A field plot experiment was conducted with

five water management regimes [Flooded, alternate wetting drying (AWD – 60 and 40)) and row (R-60 and R-40)]

using two rice cultivars (Indica and Japonica). In another field experiment, the effect of soil amendments, phosphate,

silicate and rice husk biochar on As and Cd accumulation in brown rice was investigated.

Materials and MethodsThe experimental site was located in a conventional agricultural field near Hangzhou city, Zhejiang province, east China. Two rice cultivars were compared, Zhongxiang No. 1 (A16) with low Cd accumulation in the brown rice andIndonesia (A159) with high Cd accumulation. The field was contaminated from mining effluent. Total As and Cd in soil. A field plot experiment was conducted with five water management regimes [Flooded, alternate wetting drying (AWD – 60 and 40)) and row (R-60 and R-40)] using one rice cultivar of Indica variety and another rice cultivar of Japonica variety.

1. Flood: 10 cm flood maintained during season 2. 2. AWD/40:Field flooded or 7-10 d, then allowed to dry until it reaches 40% of field capacity and then re-

flooded to 10 cm. Dried at R7 stage. 3. AWD/60:Field flooded for 7-10 d, then allowed to dry until it reaches 60% of field capacity and then re-

flooded to 10 cm. Dried at R7 stage. 4. Row/60:Planted on beds; watering by furrows until soil in beds reaches 60% of field capacity. Dried at R7

stage. 5. Row/40:Planted on beds; watering by furrows until soil in beds reaches 40% of field capacity. Dried at R7

stage.

AWD flooding the soil and then allowing to drydown before being reflooded

The total As and Cd concentrations in brown rice samples were analysed using ICP-MS after microwave digestion.

In another field experiment, the effect of soil amendments, phosphate, silicate and rice husk biochar on As and Cd

accumulation in brown rice was investigated. Application rate. Gongju field was used. Contaminated with. A rice

washing experiment was also conducted to study the effect of washing by deionized water on the removal rate of

inorganic As in polished rice. Four polished rice samples were collected from local markets and washed for three

times using deionized water. The samples were dried, finely ground and As species in rice was quantified using

HPLC-ICP-MS following hot block digestion.

Four water management treatments were laid out in arandomized complete block design, replicated three times.Treatments were: (i) Flood (continuously flooded control), (ii)AWD/40F (flood), (iii) AWD/60, and (iv) AWD/40, whereAWD represents alternate wetting and drying followed by thepercent of saturated volumetric water when fields werere-flooded. For the AWD/40F treatment water was managedthe same as the AWD/40 management until the plantsreached the reproductive growth stage; after which a floodwas maintained until the field was drained for harvest.

For the AWD water treatments, the plots wereirrigated to a flood depth of 10 cm and the water was allowedto subside via evapotranspiration and percolation until soilmoisture reached the critical moisture level for that treatment(60 and 40% of saturated volumetric water – measured at5 cm depth) when the plots were reflooded. In the Flood treatment,water was maintained at 10-cm.

Results and Discussion

Water management Compared to the flooded treatment, AWD and row treatments significantly reduced the concentration of As in brown rice with R-40 showing the least concentration. However, no noticeable differences between the rice varieties were observed. In the case of Cd, the trend was opposite; increasing Cd concentrations were noticed in row and AWD treatments compared to the flooded treatment. Between the varieties

Uptake of Cd and As has been shown to differ with rice cultivars (Norton et al. 2009; Hu et al. 2013a). Several studies have indicated that Cd uptake is significantly higher in Indica cultivars than in Japonica cultivars grown in Cd contaminated soils (He et al. 2006).

Hu et al. 2013a; b. With increasing irrigationfrom aerobic to flooded conditions, the soilHCl-extractable As concentrations increased significantlyand the HCl-extractable Cd concentrationsdecreased significantly. These trends were consistentwith the As and Cd concentrations in the straw, husk

and brown rice.The intermittent andconventional treatments produced higher grain yields

than the aerobic and flooded treatments.Yang et al. 2009. Observed that alternate wetting and moderate soil drying reduces Cd in rice grains and

increases grain yield. Recently Linquist et al. (2015) noticed Relative to the flooded control treatment and depending on the AWD treatment, yields were reduced by <1–13%; water-use efficiency was improved by 18–63%, global warming potential (GWP of CH4 and N2O emissions) reduced by 45–90%, and grain As concentrations reduced by up to 64%.

● As and Cd in rice grains● Yield● Difference in cultivars

Effect of amendmentsWhile phosphate reduced As accumulation in SK cultivar, silicate increased As concentration in both the cultivars. Biochar increased As concentration in SK cultivar with no significant change in HK cultivar, but the trend was opposite for Cd. Silicate significantly decreased the Cd accumulation only in SK cultivar.

● As and Cd in rice● Effect of amendments● Yield● Difference in cultivars

Phosphate

Silicate

Biochar

Effect of washing –arsenic – polished rice-inorganic AsThe inorganic As levels in polished rice after three washings were reduced to 37-57% of those in raw rice samples.

Cd and As concentrations in different plant partsCd concentrations in straw of both cultivars increased inall treatments as plant growth proceeded but decreasedat maturity under flooding, particularly in cultivar A159(Fig. 3). In both cultivars, the Cd concentrations instraw of the aerobic and intermittent flooding treatmentswere significantly higher than those of the control andflooded plots from panicle initiation to maturity. Incontrast to a high Cd accumulation in A159 grains,A159 straw showed a lower Cd concentration at maturitythan A16 in the aerobic and intermittent floodingtreatments. Total straw As concentrations decreased atearly growth stages but increased at the grain fillingstage of A16 with the exception of the aerobic treatmentin which the increase was observed at panicle initiation(Fig. 3). In A159, this increase was found at panicleinitiation. Overall, straw As concentrations in the fourwater regimes followed the sequence aerobic<intermittent<

control<flooded.Root Cd concentrations increased over the whole growthperiod with significantly increased concentrations in theaerobic and intermittent over the control and flooded plots(Fig. 4). The aerobic irrigation regime gave the highest rootCd concentrations followed by intermediate>control>flooded for both rice varieties. Root As concentrations

showed the opposite trend to Cd. Arsenic decreased at earlygrowth stages but increased slightly at the filling stage. RootAs followed the sequence aerobic<intermittent<control<flooded under the different water management treatments(Fig. 4).Brown rice and husk Cd concentrations showed the sametrend, i.e. aerobic>intermittent>control>flooded (Fig. 5).The flooding treatment and the control led to slightly orsignificantly higher brown rice and husk As than the aerobicor intermittent (Fig. 5). Cultivar A16 had lower Cd concentrationsbut higher As in the brown rice than A159. Arsenicconcentrations in both varieties followed the sequenceroots>straw>husk>grain and Cd gave roots>straw>grain>husk (Figs. 3, 4 and 5).

3.3 Effects of water management on grain yieldsThe different water regimes influenced the grain yields ofboth cultivars. The grain yields of both varieties in thecontrol plots were higher than the other treatments. A significant decrease in yield was observed in R-40 treatment. In other treatments, slight decrease was noticed when compared to control.those in the aerobic and flooding treatments (Fig. 6). Thetwo varieties did not differ significantly in grain yield exceptfor the flooded plots in which A159 had higher grain yieldsthan A16.

ConclusionArsenic and Cd contamination threatens food security, safety and quality, as well as the long-term agricultural sustainability of rice crops.

References

[1] Williams PN, Price AH, Raab A, Hossain SA, Feldmann J, Meharg AA. Variation in arsenic speciation and concentration in paddy rice related to dietary exposure. Environ. Sci. Technol. 39: 5531-5540 (2005)









[2] Williams PN, Price AH, Raab A, Hossain SA, Feldmann J, Meharg AA. Variation in arsenic speciation and concentration in

paddy rice related to dietary exposure. Environ. Sci. Technol. 39: 5531-5540 (2005)[1] Williams PN, Price AH, Raab A, Hossain SA, Feldmann J, Meharg AA. Variation in arsenic speciation and concentration in












paddy rice related to dietary exposure. Environ. Sci. Technol. 39: 5531-5540 (2005)

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