Characterization of soil heavy metal pools in paddy fields in Taiwan: chemical extraction and solid-solution partitioning
- First Online:
- 612 Downloads
Background, aim, and scope
Ongoing industrialization has resulted in an accumulation of metals like Cd, Cu, Cr, Ni, Zn, and Pb in paddy fields across Southeast Asia. Risks of metals in soils depend on soil properties and the availability of metals in soil. At present, however, limited information is available on how to measure or predict the directly available fraction of metals in paddy soils. Here, the distribution of Cd, Cu, Cr, Ni, Zn, and Pb in 19 paddy fields among the total, reactive, and directly available pools was measured using recently developed concepts for aerated soils. Solid-solution partitioning models have been derived to predict the directly available metal pool. Such models are proven to be useful for risk assessment and to derive soil quality standards for aerated soils.
Material and methods
Soil samples (0–25 cm) were taken from 19 paddy fields from five different communities in Taiwan in 2005 and 2006. Each field was subdivided into 60 to 108 plots resulting in a database of approximately 3,200 individual soil samples. Total (Aqua Regia (AR)), reactive (0.43 M HNO3, 0.1 M HCl, and 0.05 M EDTA), and directly available metal pools (0.01 M CaCl2) were determined. Solid-solution partitioning models were derived by multiple linear regressions using an extended Freundlich equation using the reactive metal pool, pH, and the cation exchange capacity (CEC). The influence of Zn on metal partitioning and differences between both sampling events (May/November) were evaluated.
Total metals contents range from background levels to levels in excess of current soil quality standards for arable land. Between 3% (Cr) and 30% (Cd) of all samples exceed present soil quality standards based on extraction with AR. Total metal levels decreased with an increasing distance from the irrigation water inlet. The reactive metal pool relative to the total metal content is increased in the order Cr << Ni = Zn < Pb < Cu < Cd and ranged from less than 10% for Cr to more than 70% for Cd. Despite frequent redox cycles, Cd, Pb, and Cu appear to remain rather reactive. The methods to determine the reactive metal pool in soils yield comparable results, although the 0.43 M HNO3 extraction is slightly stronger than HCl and EDTA. The close correlation between these methods suggests that they release similar fractions from soils, probably those reversibly sorbed to soil organic matter (SOM) and clay. The average directly available pool ranged from less than 1% for Cu, Pb, and Cr to 10% for Ni, Zn, and Cd when compared to the reactive metal pool. For Cd, Ni, Zn, and to a lesser extent for Cu and Pb, solid-solution partitioning models were able to explain up to 93% (Cd) of the observed variation in the directly available metal pool. CaCl2 extractable Zn increased the directly available pool for Ni, Cd, and Cu but not that of Pb and Cr. In the polluted soils, the directly available pool was higher in November compared to that in May. Differences in temperature, rainfall, and changes in soil properties such as pH are likely to contribute to the differences observed within the year. The solid-solution partitioning model failed to explain the variation in the directly available Cr pool, probably because Cr is present in precipitates rather than being adsorbed onto SOM and clay. Despite obvious differences in parent material, source of pollution, climate, and land use, solid-solution partitioning of Cd in paddy fields studied here was similar to that in soils from Belgium and the Netherlands.
To assess risks of metals in soils, both analytical procedures as well as models are needed. The three methods tested here to determine the reactive metal pool are highly correlated and either of these can be used. The directly available pool was predicted most accurately by the 0.43 M HNO3 method. The similarity of metal partitioning in paddy soils compared to well-drained soils suggests that changing redox conditions in paddy fields have a limited effect on the geochemical behavior of metals like Cd, Ni, and Zn. Small but significant differences in the directly available metal pool during the year suggest that redox cycles as well as differences in rainfall and temperature affect the size of the directly available metal pool. The large observed spatial heterogeneity of contaminant levels requires ample attention in the setup of soil monitoring programs.
The directly available pool (0.01 M CaCl2) of Cd, Zn, and Ni in paddy fields can be described well by an extended Freundlich model. For Cu and Pb, more information on dissolved organic carbon is needed to obtain a more accurate estimate of the directly available pool.
Recommendations and perspectives
Soil testing protocols and models used in risk assessment consider the availability of pollutants rather than the total metal content. Results from extensive testing indicate that approaches developed for nontropical regions can be applied in paddy fields as well for metals like Cd, Ni, and Zn. This study shows that the chemical behavior under drained conditions in paddy fields is comparable to that observed in soils across the European Union, which allows regions with large scale soil pollution including Taiwan to apply such concepts to derive meaningful experimental protocols and models to assess risks of metals in soils.
KeywordsAvailability Contamination Extraction methods Heavy metals Paddy fields Risk assessment Soil quality standards Solid-solution partitioning models
- Bruemmer GW, Gerth J, Tiller KG (1981) Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. I. Adsorption and diffusion of metals. J Soil Sci 39:37–52Google Scholar
- Brus D, Li Z, Song J, Koopmans GF, Temminghoff EJM, Yin X, Yao C, Zhang H, Luo YM, Japenga J (2009) Predictions of spatially averaged Cd contents in rice grains and associated health risks in the Fuyang Valley, P.R. China. J Environ Qual 38. doi:10.2134/jeq2008.0228
- Buekers J (2007) Fixation of cadmium, copper, nickel and zinc in soil: kinetics, mechanisms and its effect on metal bioavailability. PhD thesis no. 754, Catholic University Leuven, Leuven, BelgiumGoogle Scholar
- Clayton PM, Tiller KG (1979) A chemical method for the determination of heavy metal content of soils in environmental studies. Technical paper no. 41, Division of Soils, CSIRO AustraliaGoogle Scholar
- Colombo C, Van Den Berg CMG (1998) Determination of trace metals (Cu, Pb, Zn and Ni) in soil extracts by flow analysis with voltammetric detection. Int J Environ Anal Chem 71:1–17Google Scholar
- Daum D, Bogdan K, Schenk MK, Merkel D (2001) Influence of the field water management on accumulation of arsenic and cadmium in paddy rice. In: Horst WJ et al (eds) Plant nutrition—food security and sustainability of agro-ecosystems through basic and applied research. Kluwer, Norwell, pp 290–291Google Scholar
- EPA (2006) Regulations governing the preliminary assessment of soil and groundwater pollution control sites. Environmental Protection Administration order Huan-Shu-Tu-Tzu no.0950023629, March 29, 2006Google Scholar
- Franz E, Römkens PFAM, Van Der Fels-Klerx I (2008) A chain modeling approach to estimate the impact of soil cadmium pollution on human dietary exposure. J Food Prot 71:2504–2513Google Scholar
- Houba VJG, Van Der Lee JJ, Novozamsky I (1997) Soil and plant analysis. Part 1: soil analysis procedures. Wageningen University, WageningenGoogle Scholar
- Kikuchi T, Okazaki M, Kimura SD, Motobayashi T, Baasansuren J, Hattori T, Abe T (2008) Suppressive effects of magnesium oxide materials on Cd uptake and accumulation into rice grains II: suppression of Cd uptake and accumulation into rice grains due to application of magnesium oxide materials. J Hazard Mat 154:294–299CrossRefGoogle Scholar
- McBride MB (1994) Environmental chemistry of soils. Oxford University Press, OxfordGoogle Scholar
- Römkens PFAM, Groenenberg JE, Bril J, Vries W (2004) Derivation of partition equations to calculate heavy metal speciation and solubility in soils. Alterra, Wageningen Report no. 305Google Scholar
- USDA (1996) Soil Survey Laboratory Methods Manual. USDA, Washington, D.C. Soil Survey Investigations Report no. 42Google Scholar
- US-EPA (2002) Methods for the determination of total organic carbon (TOC) in soils and sediments, NCEA-C-1282 EMASC-001. US-EPA, Las VegasGoogle Scholar
- Xu R (2000) Particle characterization: light scattering methods. Particle Technology Series Vol 13. Kluwer Academic Publishers, DordrechtGoogle Scholar