Effects Of Municipal Sludge Compost Application On Loess Properties And Bioavailability Of Cadmium,Copper And Zinc

How to dispose dramatically increased municipal sludge?MS?effectively and safely has become a technical challenge and needs to be resolved urgently for sustainable economic and social development and ecological environment protection.Land application of MS,as an environmental friendly method for sludge reuse,is being widely used to dispose MS.It is essential to develop a sludge land application technology to adapt to local ecological environment condition,with high efficiency,little or no harm to environment,and high cost-effectiveness.In this thesis,loess,which widely distributes in the northwest of China,was used as a tested parent soil,amended with municipal sludge compost?MSC?.Typical heavy metals such as cadmium?Cd?,copper?Cu?and zinc?Zn?in MS were determined to access the feasibility and efficiency of MSC on the translocation and bioaccumulation of pollutants in MSC amended loess-plant system The effects of Cd concentration and MSC to soil ratio on loess properties and bioavailability of heavy metals were examined.The mechanisms of how MSC affects the absorption and desorption of Cd,Cu and Zn were discussed.The main contents and results of this thesis are summarized as follows:?1?The feasibility of MSC for improving loess was evaluated by matrix culture?hydroponics and soil culture?experiments,leaching experiments and pot experiments.The results showed that compared to MS,MSC is a better material for loess improvement because the extracted concentrations of Cd,Cu and Zn in MSC reduced by 33%,18%and 28%,while the plant germination rate increased by 60%.?2?Loess fertility and plant growth both promoted with increasing MSC application rates.Compared with control soil?0%treatment?,the soil organic matter,cation exchange capacity?CEC?,total nitrogen and total phosphorus increased by 790%,288%,1846%and 1077%.And the dry biomass of roots,stems and leaves,pods and beans increased by 70%,88%,213%and 275%,When MSC application rate was 30%?w/w?.The results showed that the efficiency of one-time application of 30%MSC to amend loess is the highest.The bio-concentration factor?BCF?of heavy metals in MSC amended loess was higher than that in non-amended loess,while the translocation factor?TF?of heavy metals in MSC amended loess was lower than that of the non-amended loess.Application of the sludge compost to the soil changed the physicochemical properties of loess.The changed soil properties in turn affected the interaction between heavy metal ions.The significant increase in the biomass of the aboveground parts of plants changed the distribution of heavy metals in plants.The above two factors jointly affected the bioavailability of heavy metals in the MSC amended-loess.?3?The plant height and biomass of broad bean was significantly affected by the fertility level of the amended loess,and the contents of heavy metals in the plant were significantly correlated with MSC dosage and Cd stress level.Under exogenous Cd stress(2¹8 mg kg^-1),the activity of Cd in the amended loess increased,and the broad bean plants had the BCF_Cd>1,Cadmium may enter the plant root through the apoplastic transport from soil.However,the TF_Cd of faba bean plant in all treatments was less than 1,which was related to the physiological regulation mechanism of broad bean plant,the dilution effect of biomass and the antagonism between Cu²⁺,Zn²⁺and Cd²⁺.The distribution coefficient of heavy metals in broad bean plant indicated that the application of MSC can help the plant root to intercept heavy metals,intercept rate?75%.Hence,after the intake of broad beans,the exposure levels of Cd,Cu and Zn in human body will be much lower than the recommended values of Provisional Tolerable Daily Intake?PDTI?and Chine Dietary Reference Intakes?CDRIs?.However,the human health risk level will be increased with the increase in Cd stress level.Therefore,it is necessary to monitor Cd concentration in both soil and plant when amending loess by MSC.?4?The particle size of loess,the types of extractants and the extraction time all have influenced on the leaching efficiency of heavy metals in loess.Among them,Ethylenediaminetetraacetic acid disodium salt?EDTA-2Na?had the highest extraction efficiency for Cd and Zn under 0.15 mm particle size for 90 min,while Diethylenetriaminepentaacetic acid?DTPA?had the best extraction efficiency for Cu under 0.85 mm particle size for 90 min.The leaching rates of Cd,Cu and Zn were40.69%,82.17%and 95.47%.The capacity of loess to retain heavy metals decreased in the following order:Cd>Zn>Cu.With Cd²⁺initial concentrations(0,1.5,5,10,18,30 mg L^-1)increasing,both the adsorption and desorption capacity of loess for Cd²⁺increased,and Cu²⁺and Zn²⁺release rate increased.After addition of the dissolved organic matter?DOM?which extracted from MSC to loess,the absorption of Cd²⁺was decreased,but the efficiency of Cd²⁺,Cu²⁺and Zn²⁺desorption was increased.With the increase in MSC dosage,the contents of total organic carbon and heavy metals in the amended-loess DOM increased,while pH slightly decreased??pH=0.13?.Removal of DOM from the amended loess increased the adsorption capacity of the amended loess when Cd²⁺concentration was low,but decreased the adsorption efficiency when Cd²⁺concentration was high.Fourier transform infrared spectroscopy?FI-IR?results show that the functional groups in amended loess made no contribution to the adsorption of Cd²⁺.The results of new diffraction peak?2?=27.48°,27.56°?analysis of X-ray diffraction?XRD?clay minerals showed that the complexation reaction between clay minerals and Cd enhanced the adsorption ability of loess to Cd.In summary,amending loess with MSC is an ideal way of improving its function for plant growth.The dosage of MSC application and Cd stress levels had significant influence on plant growth and bioavailability of heavy metals,and the influence effect was closely related with the changes of loess properties?pH,organic matter and clay minerals,DOM etc.?.Cd was a limiting factor in the process of amending loess with MSC,so it is necessary to strictly monitor Cd concentration in amended loess.