| Land application is presently the most promising method for sludge disposal because it brings sludge back into the circulation of substances and energy in the natural environment to enhance soil quality and promote plant growth.Yet,there are numerous restrictions on large-scale sludge land application,especially the heavy metals in sludge that will pollute the natural environment.Attapulgite is a chain-like hydrated magnesium aluminosilicate mineral,which is characterized in its nano-scale lattice structure and developmental channels in the fibrous structure.Meanwhile,with large specific surface area and special surface charge distribution,attapulgite is relatively efficient in the absorption of heavy metal ions.Being applied to sludge,attapulgite is expected to modify the physicochemical properties with its special structure and adsorbability,change the chemical forms of heavy metals in sludge,adsorb and immobilize the heavy metals in sludge so that the heavy metals in sludge have lower mobility,bioavailability and ecological toxicity while the immobilized sludge imposes a lower environmental risk to resource-based land application.In this study,the attapulgite originating from Linze,a county of Gansu province,was used in the experiment of immobilizing the heavy metals in the sludge discharged by the Qilihe-Anning waste water treatment plant in Lanzhou.In the immobilization and culture experiment,attapulgite was added to sludge to explore the impacts of the dose of attapulgite and the time of immobilization on physicochemical properties of sludge,chemical forms and bioavailability of heavy metals in sludge and probe into the immobilization effect factors and the time-effect relationship;a biological receptor was used in a toxicity experiment to analyze the ecological toxicity of the immobilized sludge and then a pot experiment was carried out to observe the influence of applying the immobilized sludge to soil on the enrichment of heavy metals in corns.The study results provided a scientific basis and technical guidance for immobilization of heavy metals in sludge.The main findings are as follows:1.Attapulgite had significant impacts on the physicochemical properties of sludge.Adding attapulgite increased potassium content and balanced nutrients in sludge,making urban sludge a potential source of organic fertilizer.As the dose of attapulgite increased,the pH value of sludge rose significantly and reached the peak(pH = 8.02)at the maximal dose of 30%.As the immobilization of sludge continued over time,the pH values of different sludge samples decreased slightly while the degradation of organic matter in the immobilized sludge became slower.In the process,the fulvic acid(FA)in the organic matter dropped significantly;in contrast,the humic acid in the organic matter increased remarkably.2.Attapulgite plays an important role in changing the chemical forms of heavy metals in sludge.In the sludge treatment without attapulgite,Cu,Cr,Ni and Zn were highly active.After attapulgite was applied to the sludge samples,the acid soluble,reducible and oxidizable fractions of Cu,Cr,Ni and Zn were all converted into residual fractions.Moreover,as the dose of attapulgite increased,greater conversion ratios were observed.The acid soluble fractions of Cr and Ni in each sludge treatment appeared to decline most steeply while the oxidizable fractions of Cu and Zn had the largest loss;the residual fractions of Cu,Cr,Ni and Zn achieved enormous increase.The pH value and humic acid(HA)content of sludge are closely associated with the changes in the forms of heavy metals in sludge.As the pH value and HA content increase and the mineralization of organic matter slows down,heavy metals in sludge are immobilized at a higher speed.The evaluation of sludge stability suggests that attapulgite can improve the stability of the four types of heavy metals in sludge.Throughout the immobilization process,the heavy metal Cr in different forms remained stable while Zn,Cu and Ni in the forms of activity and potential activity were reduced slightly as the immobilization continued.3.Attapulgite was effective in reducing the bioavailability of heavy metals in sludge.As the dose of attapulgite increased,the water-soluble metals,plant-available metals(DTPA extraction)and those leached out through the extraction procedure for leaching toxicity declined sharply.Significant differences were found among extraction methods in the performance of extracting different types of metals.After sludge treatment,the water-soluble fractions of Cu and Ni dropped most dramatically,which were reduced by over 70% compared to the sludge treatment without attapulgite(SS)when the dose of attapulgite reached 30%(SA6).The DTPA extractant yielded the highest extraction ratio of Ni,equal to 14.4-20.5% of the total Ni content.Besides,water-soluble fractions of Ni also surpassed those of the other three heavy metals.Therefore,Ni showed a relatively high degree of bioavailability.The heavy metals extracted by DTPA can be sorted by content: Zn > Ni > Cu > Cr,whereas the humus-metal ion complex stability constants are arranged in the exactly opposite order,i.e.,Cr > Cu > Ni > Zn.In the sludge samples,the extracted leaching toxicity in Cu was declined most significantly.Compared to SS,it was decreased by 60.11% in the SA6 treatment.The immobilization efficiencies of the four types of metals can be described as Cu > Ni > Cr > Zn,which have significant correlations with the electronegativities of heavy metal ions.With the increase of passivation incubation time,except for the slight increase of the plant-available Cu by 2-6%,other plant-available metals and those leached out through the extraction procedure for leaching toxicity maintained stability.4.Attapulgite lowered the ecological toxicity of sewage sludge effectively.The extractive in the SS treatment had significant inhibition on against Chinese cabbage seed germination,with the germination index(GI)of 27.6%.Apart from this,it also had toxicity to earthworms.As the dose of attapulgite in sludge increased,the Chinese cabbage seed germination rate and the root growth rose sharply while the survival rate and weight of earthworms also grew to a certain degree.After adding attapulgite,the relative seed germination(RSG),relative root growth(RRG)and germination index(GI)were respectively increased to 88.9%,84.0% and 72.9% from 49.7%,55.9% and 27.6% in the SS treatment.When the dose of attapulgite was greater than or equal to 15%,the weight of earthworms growed by about 4%.Among the four types of heavy metals,Ni had the highest toxicity to Chinese cabbage seed germination and the growth of earthworms whereas Cu had the lowest toxicity.5.Attapulgite could be applied to soil with sludge in order to prevent and control enrichment of heavy metals in maize.The accumulation values of four heavy metals in stems,leaves,roots and kernels of maize all showed a decreasing trend with the increasing dosage of attapulgite;in different soil treatment,the heavy metals contained in the corn roots were richer than the overground parts,followed by stems and leaves.In contrast,the grains had the least content of these heavy metals.The enrichment and transport coefficients of corn stems,leaves and grains were all smaller than 1.Among all sludge treatments,the biologically available metals had significantly positive correlations with the metal content in different parts of corns. |
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