Effects Of Heavy Metal Ions On Removal Of Nutrients And On Release Of Dissolved Organic Matter By Duckweed Systems From Anaerobically Digested Swine Wastewater

In the few years,heavy metals including zinc ions(Zn²⁺)and copper ions(Cu²⁺)have also been used as feed additives in many feed factories and livestock farms to promote the growth of livestock.It leads to in high concentrations of heavy metal ions in anaerobically digested swine wastewater（ADSW）.Furthermore,high concentration of heavy metal ions can inhibit various microorganisms and plants,which pose serious challenges to traditional treatment methods including microbiological treatment and phytoremediation for ADSW.Recently,many investigations focused on the effect of heavy metal ions on microbial activity for swine wastewater treatment.However,few reports were available on the effect of Zn²⁺and Cu²⁺on duckweed for ADSW treatment and on release of dissolved organic matter（DOM）in duckweed systems.Therefore,this research focused on the effects of effect of Zn²⁺and Cu²⁺on removal of nutrients from swine wastewater and on release of dissolved organic matter in duckweed systems.These data and results are supposed to be referred in design,optimization and operation of phytoremediation processes for treatment and recycling of ADSW contaminated by heavy metals using duckweed.In this paper,the synthetic ADSW and Lemna aequinoctialis were selected for a batch experiment,and the concentrations of Zn²⁺were ranged from 0 to 15 mg/L as well as the concentrations of Cu²⁺were ranged from 0to 5.0 mg/L.The main research contents and results were as follows:Firstly,the effects of Zn²⁺concentrations on removal of NH₃-N and TP by Lemna aequinoctialis and on fresh weight,protein of Lemna aequinoctialis were investigated for treatment of ADSW.Results showed that the optimal Zn²⁺concentration for NH₃-N and TP removal,duckweed growth and the accumulation of proteins was 5.0 mg/L.The higher（>10mg/L）or lower（1.0 mg/L）Zn²⁺concentrations would inhibit the nutrient removal,duckweed growth and the accumulation of proteins by Lemna aequinoctialis from synthetic ADSW.Secondly,the effect of Zn²⁺concentrations on assimilation of Zn²⁺by Lemna aequinoctialis was evaluated.Results showed that no obvious change was observed under the initial Zn²⁺concentrations of 0-5.0 mg/L in the ADSW.While the initial Zn²⁺concentrations were ranged from 10-15 mg/L,the concentrations of Zn²⁺were decreased in the ADSW.Thirdly,the effect of Zn²⁺concentrations on photosynthetic pigments and vitamin E of Lemna aequinoctialis were analyzed.Results showed that the higher the concentration of Zn²⁺,the more obvious inhibition of the synthesis of photosynthetic pigments on Lemna aequinoctialis.Moreover,the high concentrations of Zn²⁺would promote the high-yield vitamin E of duckweed from ADSW.Fourthly,the effect of Cu²⁺concentrations on removal kinetics of NH₃-N and TP from duckweed was investigated.Results showed that the optimal Cu²⁺concentration for NH₃-N and TP removal was 0.96 mg/L via the analysis of the new kinetic model.The removal rate of NH₃-N and TP could be improved at 0.1-1.0 mg/L of Cu²⁺,while was inhibited at 2.0-5.0mg/L of Cu²⁺in duckweed systems.Fifthly,the effect of Cu²⁺concentrations on the fluorescent components of DOM released from the duckweed systems was evaluated.Results showed that there were four components including microbial by-product,tyrosine-like,NADH,and humic-like substance in DOM released from the duckweed systems through the excitation-emission matrix-parallel factor（EEM-PARAFAC）analysis,and the NADH exhibited high capacities of binding copper ions.Besides,the principle component analysis（PCA）showed that the dominant DOM were lower molecular weight compounds at 1.0 mg/L of Cu²⁺and higher molecular weight compounds at 2.0-5.0 mg/L of Cu²⁺.Sixthly,the effect of Cu²⁺concentrations on the molecular characteristics of DOM released from the duckweed systems was explored.Results showed that The stress of Cu²⁺on the binding order between the function groups of DOM and copper ions followed the order as:C-H,N=O,Ar-H>N-H,COO^->C-O-C>C-O according to the results of the the result of the two-dimensional Fourier transform infrared correlation spectroscopy（2D-FTIR-CoS）analysis.Moreover,the bonds of C-H,N=O and Ar-H in DOM were responsible for the variations of DOM conformations at a critical concentration of 0.5 mg/L Cu²⁺from the perturbation correlation moving window two-dimensional（PCMW2D）analysis.