## 4 Discussion  
This study examined the roles of plants in CWs for removing RY86. CWs planted with cattail and papyrus demonstrated much higher removal for RY86 than the unplanted CW. The gravel packed in the CWs adsorbed only a small amount of RY86 in wastewater. RY86 is hardly degraded by photolysis under usual conditions [15; 16; 17]. Reportedly, the presence of common reed did not affect the removal of dyes [14]. However, the results obtained in this study indicate the importance of selecting plants for treating dye-containing wastewater. Cattail and papyrus were actually planted in CWs in Pekalongan city for treating Batik wastewater [1]. The RY86 removal in the planted CWs increased with HRT from 1 day to 15 days. Earlier studies also presumed HRT of 1.5-15 days in CWs for the treatment of textile wastewater [12; 15]. The relation between the evapotranspiration of wastewater and the RY86 removal in the CWs is summarized in Figure 7. Root development is related to the efficiency of uptake of water, nutrients, and RY86. Although a large amount of evapotranspiration occurs in the CWs with long HRTs, few studies have evaluated its effects on dye uptake by plants. For evaporation of 2.5-22.5%, RY86 removal in the unplanted CW was less than 23%. With increased evapotranspiration, the RY86 removal increased drastically, suggesting that RY86 was uptaken by the plants. Over the same range of the evapotranspiration, the Papyrus-CW showed higher RY86 removal than that of Cattail-CW. The maximum RY86 removal values in the Cattail-CW and the Papyrus-CW were, respectively, 83% and 91%, with evapotranspiration of 62% and 47%. Greater efficiency of C. _payyrus_ was also reported for treating municipal wastewater than that of _P. australis_[21] and for removing heavy metals and enteric bacteria than that of broad leaf cattail _Typha latifolia_[22]. Reportedly, RR2 (MW = 615) with a small molecular size was taken up easily by burhead [11]. With molecular size as small as that of RR2, RY86 (MW = 667.37) might be easily uptaken by cattail and papyrus. Detoxification pattern of xenobiotic molecule depends upon the plant species [23]. Detailed information about inherent pathways of the plants to metabolize the toxicants is lacking [24]. Further studies of accumulation, degradation, and toxic effects of RY86 in the plant tissues must be conducted to elucidate the fate of the dye in the CWs. The mineralization efficiency for RY86 in the CWs remained unknown. The higher removal for RY86 than the evapotranspiration ratio (Figure 7) suggests that the dye was removed by degradation by microorganisms in addition to the plant uptake  
Figure 7: Evapotranspiration effects on RY86 mass removal in CW-treated wastewater. The dashed line indicates the RY86 removal by absorption along with water. (Figure 6). Root exudates of plants can grow microorganisms in their rhizosphere. Azo dyes can be degraded by azo reductases for the reducing azo bond (-N=N-) by anaerobic/ anoxic bacteria [25] and lignin peroxidase, lacases, and peroxididases of fungi [26; 27]. However, few studies described decolorization specific for RY86 by microorganisms [20]. Reportedly, _B. safensis_ isolated from earthworm gut can degrade RY86 [18]. Phylogenetic identification and characterization of RY86-decolorizing microorganisms found in this study (Figure 6) will be helpful for further elucidation of decolorization mechanisms in the CWs. The biodegradation pathway of RY86 by microorganisms should be elucidated because toxic aromatic amines can be accumulated in the azo-dye degradation process [28; 29]. The organic matter in wastewater used for this study was only RY86, which is hardly biodegradable. Addition of easily and slowly degradable organic matter might enhance the growth of dye-degrading microorganisms. Reportedly, application of rice agricultural waste to CW with inoculation of a dye-degrading _Psychrobacter alimentarius_ strain and planted with _Presaria barbata_ can enhance removal of Reactive Black 5 [14].