# Combined Effect of Ferrous Ion and Biochar on Cadmium and Arsenic Accumulation in Rice
## 1 Introduction  
Anthropogenic activities and mining are major sources of metals accumulation in soils, especially in agricultural soils [1, 2]. The pollution of Cd in paddy soils is a widespread problem due to its potential harm to plants and human health [3]. Rice (_Oryza sativa_ L) is a widely cultivated crop in China [1], and also a common dietary source for Cd and As exposure. A number of studies have reported that metal contents exceed safe values [4]. Cultivated in flooded soil, with the enhanced bioavailability of As in soil due to the anaerobic conditions, the rice take in more [5]. In addition, As is more easily taken in rice than other cereals [6]. Owing to high redox potential, aerobic conditions in paddy soil can obviously inhibit the uptake of As in grains, whereas rice growing tends to increase uptake of Cd in rice under the aerobic conditions [7, 8]. For ensuring the grain safety, how to simultaneously reduce the uptake of Cd and As in rice has become a vital issue and drawn much public attention.  
Biochar is a carbon-rich material produced by the thermochemical conversion of biomass in a anoxic-high temperature condition, commonly using biomass materials including agricultural waste,animal and plant residues, etc. [9]. In recent years, many researchers have highlighted that the biochar as a novel soil remediation or amendment was effective in remediating inorganic contaminants [10; 11]. Agricultural solid waste has been widely used as a source of biochar materials [12; 13]. The treatments with biochar reduce Cd transferring from soil to rice owing to its surface absorption and functional groups (e.g., \(-\)OH) complexation in order to immobilize metal cations in soils [14; 15; 16]. However, biochar did not show a consistent effect in As contaminated soil, nor did it effectively reduce the content of As in rice. The chemical properties of Cd and As in soil are usually opposite, so it is a challenge to find a composite soil amendment for eliminating the content in rice as well as ecological risk for both contaminants. There are numerous iron-based materials that have been proved to be effective in the immobilization of As in soil. Zero valent iron, iron (hydr) oxides have been extensively reported on immobilizing As in contaminated soils [17; 18]. In addition, the effect of soluble Fe on reducing the bioavailability of As is better than that of metallic Fe [19]. The possible reason is that amorphous iron oxides with As can form Fe-As compounds on its surfaces with low solubility [20]. These reports suggest that decreasing As bioavailability and bioaccessibility in soil by applying Fe-material may be an effective method to immobilize As. In addition, Fe(II) and Fe(III) salts are considered to be a more effective amendments for As contaminated soil because of their chemical reactions in the soil that are more effective than iron oxides [18; 21; 22]. Therefore, Fe(II) as the main remediation material was proved to be effective in reducing the water-soluble As in soil [12; 23].  
Combining Fe-base materials and biochar to amend Cd and As co-contaminated soil have been reported, such as Fe(II) combination with compost, as a promising approach to deal with metals co-existing in high concentrations in soil [24; 25]. However, in the paddy soils system, soil functions and services should be improved while considering the mitigation of metal toxicity. Therefore, based on these characteristics and our previous studies [12; 26], biochar and ferrous ion were chosen to be amendments settling the Cd-As contaminated paddy soil, in order to figure out the disposal procedure of the materials and the translocation efficiency reduction of the two heavy metals mentioned above in contaminated rice plant soil. The objects of this study were (1) to investigate the effect of the biochar and ferrous ion on the bioavailable of Cd and As in contaminated soils and metals uptake by rice and (2) to explore the possible mechanism using biochar and ferrous ion immobilization As and Cd in soils.