Ability Of Selected Wetland Plant Species To Zinc And Cadmium Toxicity And Their Effect On Pollutant Removal

Ipomoea aquatica Forsskal and Eichhornia crassipes were used in the designed experiments with objectives to determine the ability of the selected plant species in resistance to toxicity of Zn2+ and Cd2+ ions in both single and mixed ionic species aqueous culture systems. Based on the determined upper resistance limit for respective metal ions, the experiment was further designed to study the effect of Zn2+ and K+ ions present in relatively high concentration levels in the mixed system on Cd2+ ion uptake by tested plants at their toxic Cd2+-concentration limit.A pilot constructed wetland system with integrated functions of wastewater treatment was established to investigate its efficiency to treat sewage mixed partially with lab wastewater. In the pilot system the ability of selected wetland plant species to remove nitrogen and phosphorus were compared under different aeration conditions.Results obtained from single metal ion (Zn2+, Cd2+) aqueous culture system showed that:1) Ipomoea aquatica Forsskal could resist higher levels of the tested metal ions than Eichhornia crassipes;2) The upper concentration limits for normal growth of Ipomoea aquatica Forsskal were found to be 100 and 10 mg·L-1 for, respectively, Zn2+ and Cd2+ ions while those limits for normal growth of Eichhornia crassipes were only 5 mg·L-1 for Zn2+ and 1mg·L-1 for Cd2+ ions.Results obtained from mixed metal ions aqueous culture system showed that:1) At the above determined toxic Cd2+ ion concentration limit for Ipomoea aquatica Forsskal (namely, given Cd2+=10 mg·L-1 in the mixed ionic species system), the upper concentration limits for Zn2+ and K+ ions in the mixed Cd2+/Zn2+/K+ ion system for normal plant growth were found to be Zn2+≤80 mg·L-1 and K+≤200 mg·L-1, respectively, while that for K+ ion in the mixed Cd2+/K+ ion system was, K+≤500 mg·L-1. At the determined toxic Cd2+ ion concentration limit for Eichhornia crassipes (namely, given Cd2+=1 mg·L-1 in the mixed ionic species system), the upper concentration limits for Zn2+ and K+ ions in the mixed Cd2+/Zn2+/K+ ion system for normal plant growth were found to be Zn2+≤5 mg·L-1,K+≤50 mg·L-1, respectively, while that for K+ ion in the mixed Cd2+/K+ ion system was, K+≤110 mg·L-1.2) The concentration of Cd2+ ion in both tested plant species was higher in mixed Cd2+/K+ ion systems than in single Cd2+ ion system, showing that the addition of K+ ions into the culture solution enhanced Cd2+ ion concentration in plants, The difference in Cd2+ ion concentration in both plant species, however, was not found to be significant between single Cd2+ and mixed Cd2+/Zn2+/K+ ion systems, due to, very likely, the effect of competition in plant uptake between Zn2+ and Cd2+ ions.Results obtained from the pilot study conducted in the established constructed wetland system for treatment of sewage and lab wastewater showed that:1) Water quality index of the wastewater after the treatment reached its allowed discharge standard of GB18918-2002.2) The pollutant removal rate was found to be much higher in wetland cells with plants than those without plants.3) The observed treatment efficiency varied with plant species, filler types and their combinations:a) The best combinations in primary treatment cells for removal of CODcr, TN, NH4+-N, TP were found to be, respectively, zeolite plus Pontederia cordata and Rosa chinensis, zeolite plus Thalia dealbata, zeolite plus Thalia dealbata, and Red soil plus Typha angustifoli;b) For removal of CODcr the best combinations were found to be zeolite plus Pontederia cordata, Rosa chinensis and Ligustrum X vicaryi in the primary treatment system followed by zeolite plus Lythrum salicaria, ilex chinensis and Cuphea ignea in the secondary treatment system. For removal of TN and NH4+-N the best combinations were found to be zeolite plus Thalia dealbata in the primary treatment system, connected to zeolite plus Cyperus slternifolius and Iris ensata Thunb in the secondary treatment system; For removal of TP the best combinations were found to be red soil plus Canna indica in the primary treatment system, associated with red soil plus Variegated canna and Alisma orientale in the secondary treatment system.4) Aeration with purpose to improve the oxygen condition for plant root growth was found to be effective in pollutant removal. Application of the aeration technique enhanced the removal rate of CODcr, TN, NH4+-N and TP, reduced the content of MDA in plant leaves and increased the level of bio-activity in the rhizosphere.