Distribution And Ex Situ Bioremediation Of Heavy Metal Contamination In Urban River Sediment

Pollution associated with metals is a serious problem because of their toxicity,persistence, and nondegradability in the environment. Over the past century, thelevels of metals in aquatic ecosystems have increased drastically as a result of rapidindustrialization and urbanization. The metals have accumulated in sediments. Assediment is the main sink for various pollutants, including metals, sediment qualityhas been considered as an important indicator of water contamination. Althoughthere are many researches on the sediment contamimated by heavy metals, fewapplication and study on ecological restoration of sediment has been carried out.Therefore, this paper developed the study on the distribution, sources, andecological risk assessment of heavy metals in surface sediments, screen thecandidate plant variety for bioremediation in sediment, based on the fieldinvestigation and laboratory analysis. Method of molecular biology technique wasused to evaluate the effects of ecological restoration and investigated the mechanismof plant-microorganism combined remediation. The results obtained here could notonly provide the reference for the future bioremediation in urban river sediment, butalso be used in the exploration of strategies to protect human health and theecosystem.This paper was conducted in order to determine the degree of metal pollution inthe surface sediment of an urban river across residential and industrial zones ofHarbin, China. Six sampling sites located at Xinyi river were chosen for collectingthe surface sediment samples. The sediment samples have been subjected to a totaldigestion technique and analyzed by inductive coupling plasma mass spectrometry（ICP-AES） for metals including Pb, Zn, Cd, Ni, Cr, Cu and As and AtomicFluorescence Spectrometer （AFS-2202E） for Hg. The average concentrations of As,Cu, Zn, Pb, Cr, Hg and Cd determined in the surface sediments are higher than theclassⅠstandard of Marine Sediment Quality. And the average concentration of Ni inthe surface sediments is higher than the maximum accepted level of River SedimentQuality of Hong Kong. It revealed that the surface sediments in all the samplingsites were polluted with As, Cu, Zn, Pb, Cr, Hg, Cd and Ni, especially in site D.According to the pollution assessment of heavy metal in sediment samples, thesurface sediments were contaminated with As, Cu, Zn, Pb, Cr, Hg, Cd and Ni, andthe Cd pollution was most serious. Cd poses the highest ecological risk in thesediments. While site D was the most seriously polluted among sampling sites. Thecontamination degree of heavy metals in surface sediments were in the order of Cd> Zn> Pb> Cu> Ni> Cr> Hg>As, and site F>site D> site C> site B> site A>site E. It was found that the primary inputs of heavy metals were anthropocentric disturbance, mainly including the effluent of industrial and municipal wastewater.While, Hg mainly derived from atmospheric pollution. The potential ecological riskindex （RI） and health risk assessment had been calculated. The potential ecologicalrisk of heavy metals in surface sediments were in the order of Cd> Hg> Pb> Cu>Ni> As>Zn>Cr, and1the the total health risk were in the order of As> Cd> Cr（Ⅵ）> Pb>Hg> Ni> Cr（Ⅲ）> Zn> Cu.According to the results of fractionation profile, the mobility andbioavailability of heavy metals were in the order of Cd> Pb>Zn>Ni>Cr>Cu>Hg.Based on the physicochemical properties of river, sedimentation, release anddistribution of heavy metals, dynamic models of heavy metals in polluted river wereconstructed:In water:V_w dCw/d_t=_Vs（C_s-C_s’）k_e+V_w（C_-C-w）k_h-V_w（C_w-C_j）k_j+v_h（C_h-C_w）-v_j（C_w-C_j）-V_w（C_w-C_w’）k_sAnd in sediment:V_S dC_s/dt=V_w（C_w-C_w’）k_s+V_s(C_hs-Csk_h’-V_s(C_s-C_js)k_j’-V_s（C_sC_s’）k_e+S_i18dominant species of plant, belonged to7families, were found in30plots inmud along the riverbank. RDA was introduced to analyze relationship betweenvegetation community and heavy metals in rhizospheres. As the results, the heavymetal tolerant species could be classified into4groups by the clustering analysis,such as high contamination tolerance, moderate contamination recovery, recoveryand As contamination recovery. And12metal-tolerant plants were screened. Presentresearch conducted field and pot experiments on dominant plants to find out theirphytoextraction potential. Results indicated the uptake of metals by plants showedspeaciality in different species. Solanum nigrum had a greater accumulation oncadmium （Cd） than other plants. High biomass, wide spread root system and heavymetals tolerance make S. nigrum as an attractive choice for the remediation ofsediment highly contaminated. Rumex dentatus was found a lead （Pb）-accumulatingplant. While Polygonum lapathifolium was the potential material forphytoremediating Cd in highly metals polluted condition. R. dentatus and Ixerischinensis were good Zn remediation candidates in contamination soils, but I.chinensis only suitable for moderate contamination. These other plants were themetals excluders or mainly held in the roots. The results of pot experiment showedthat R. acetosa L., S. nigrum L. and P. lapathifolium L. could increase activities ofheavy metals and bacterial number significantly. The bioremediation of heavymetals polluted sediment should adopt plant-microorganism combinated remediationand rational multi-cutting management.The statistical models of7heavy-metal phytoavailablities were constructed, taking a series of physicochemical properties of sediment samples and plants intoconsideration. The models of heavy-metal phytoavailabilties showed there wascorrelation between phytoavailabilty and physicochemical properties of sedimentsamples and other metals. phytoavailability of Cu was affected by total-Cdsignificantly. The model expressed in terms of total WI could bemathematically described bylgwI_p=0.888lgwI_HM+0.886lgCS-0.098lgOM-1.21lgCEC-2.250lgpH+2.267（R=0.915，F=49.375，n=54，p<0.001）.The difference among sediment samples and five rhizospheric samples ofdominant plant species, R. dentatus, S. nigrum, Cynodon dactylon, P. lapathifoliumand Populus canadensis, screened from field investigation were indicated andanalyzed by DGGE and16S rDNA libraries. Among the combined81OTUsidentified among S. nigrumand P. canadensis libraries, with the remaining21OTUscommon to both libraries. But there were few common OTUs among sediment andthe rhizospheric libraries. The highest relative amount of phylotypes and clones inboth rhizosphere libraries was observed for Betaproteobacteria. Results of16SrDNA sequence analysis showed metal resistant and plant growth promotingbacteria accounted for a considerable proportion in both rhizosphere libraries. Whilerepresentatives of the Actinobacteria and Firmicutes were the most numerousphylotypes in sediment clone library.