## 1 Introduction  
Soil pollution is becoming more and more serious with the rapid development of industry, sewage sludge disposal, metal processing, and energy production. Hence, it is important and urgent to remediate polluted soil. The conventional remediation methods such as the replacement of contaminated soil with clean soil, are usually expensive and intrusive to the ecosystem [1]. The phytoremediation of heavy metals, which has the advantages of being low-cost and environmentally-friendly and resulting in minimal soil disturbance, has attracted more and more interest in recent years [2; 3; 4; 5]. Heavy metals are of major environmental concern because they are non-biodegradable, and thus persist almost indefinitely in the environment. Cadmium (Cd) is one of the most toxic heavy metals, which has been classified as a human carcinogen and teratogen [6]. It is the byproduct of zinc mining, where it is found as cadmium sulfide [7]. Cd accumulates mainly in the kidneys and liver in humans and can cause many diseases such as hypertension, diarrhea, and stomach pains [8; 9]. Pitytoremediation makes use of the harvestable part of the plants to remove pollutants. Any plants can theoretically uptake metals from the soil; however, the overwhelming majority of plants generally remove only a small percentage of heavy metals from contaminated soil due to their low biomass and/or weak translocation from root to shoot. Therefore, using plants with high biomass yields, along with adding chemically enhanced agents, has been widely applied as a viable strategy for removing heavy metals from soils over a reasonable time frame [10; 11; 12]. In this study, two kinds of plants (i.e., _Taraxacum officinal_ and _Gazania_) were investigated based on a report of the effectiveness of phytoremediation of Cd by Asteraceae plants [13]. It is known that _Gazania_ can grow wild naturally without depending on the soil environment. Thus, it is considered that _Gazania_ can accumulate a large amount of Cd at the shoot because its flowering time is long. Meanwhile, _Taraxacum officinal_ is a relatively imminent plant with longer roots in which higher amounts of non-ishment can be stored, which can be expected to accumulate a lot of Cd. In this work, zinc (Zn) was also selected as a subject element because Zn pollution is known to occur at the same time near mined land where Cd pollution takes place. The objective of this research was, first, to confirm the effectiveness of _Taraxacum officinal_ and _Gazania_ for the phytoremediation of Cd and Zn by applying biodegradable chelating agents. Generally, it is well known that ethylene-diamine-tetraacetic acid disodium salt (EDTA) is as an effective chelating agent for assisting phytoremediation by enhancing the mobility of metals in soil (e.g., [14; 15]). However, it has been suggested that EDTA is a potential risk to groundwater and drinking water due to its remobilization of metals from sediments and soil [16; 17]. In order to reduce the potential environmental risk of EDTA, an easily biodegradable chelating agent, namely, (S,S)- ethylenediamine-N,N'-disuccin acid trisodium salt (EDDS), was used as a substitute for EDTA in this work based on previous literatures [18; 19]. In addition to EDDS, 3-hydroxy-2,2'-iminodisuccin acid, sodium (HIDS) and N,N-dicarboxymethyl glutamic acid (GLDA) were also used as biodegradable chelating agents. Furthermore, another purpose was to ascertain if the effect of the phytoremediation varies with the type and amount of added chelating agent. The translocation factor from root to shoot, as well as the amount of uptake, were also studied to determine which chelator represents the most effective tool for the efficient phytoremediation of Cd and Zn. It is also important to investigate the disposal method of plants after phytoremediation. Thus far, plants containing heavy metals have been generally subjected to incineration disposal [20]. However, it is desirable to recover and reuse the adsorbed materials from an industrial and technological point of view.