Hydrothermal Synthesis Of Environmental Mineral From Soil And Its Immobilization Of Brownfield Heavy Metals

The rapid industrialization accelerated urbanization.So that a huge influx of people into the cities resulted in the higher population and much scarcer land resources in China.With the economic growth,more attentions was paid to the quality of life and environment.In recent years,tens of thousands of manufacturing enterprises and factories have been forced to shut down or relocate,leaving a huge number of polluted sites（brownfields）.The polluted sites are usually in the heart of city,which not only are seriously threatening the surrounding environment and local people’s health,but also inhibit the urbanization and economic development.Because the legacy of polluted sites in the cities are mainly polluted by heavy metals,remediation/treatment of heavy metal polluted sites has to be settled urgently in China.Because the soil remediation started relatively late in China,most polluted sites have not been effectively remediated/treated.The existing soil remediation technologies cannot meet the requirement of remediation of the polluted sites with various types,and the imported foreign technologies are always“acclimatized”.In brief,it is urgent to develop a novel remediation technology for polluted sites,which is rapid,efficient,stable and environmentally friendly.Based on the element-recycling phenomenon in nature,the study simulated the diagenetic process underground,and developed a novel remediation technology of in-situ encapsulation of heavy metals via hydrothermal rapid dissolution/restructuring of Si and Al in heavy metal polluted soil into environmental mineral of aluminosilicate（zeolite）.1.To avoid the effect of clay minerals,organic matters and various impurities in soil on encapsulating/immobilizing heavy metals by the new formed zeolite during hydrothermal conversion,model soil of chemicals（SiO₂+NaAlO₂）was used for hydrothermal synthesis at first in this study.The result suggests that zeolite P could be synthesized from model soil with the Al/Si ratio of 0.3-1.0,and convert into sodalite with the Al/Si ratio higher than 1.0.The treating temperature of 160℃-200℃or treating time of 16 h-48 h was favorable to zeolite P formation,while too high temperature or too long time could make the formed zeolite P transform into analcime.To study the characteristic of in-situ immobilization of heavy metal and the microstructure behavior of zeolite P during immobilization,Cr,Zn and Cd were chosen for immobilization due to not only they are common in the polluted soil but also their different sizes and charges(62 pm of Cr³⁺,74 pm of Zn²⁺and 95 pm of Cd²⁺).The result demonstrates that the additive/immobilization of heavy metals not only inhibited the dissolution/restructuring of model soil,but also delayed the formation and growth of zeolite P,weakened its crystallinity.The higher the charge density,the stronger the ligand effect of heavy metals have,the more heavily the retard/delay with the order of Cr>Zn>Cd.EDS and Rietveld refinement analyses further confirmed the selectivity of encapsulation of heavy metal by zeolite P.The encapsulated heavy metals occupied the original Na positions（Na1 and Na2）of8-member-ring channel selectively because of their different cation size and charge density.Because Na1 is closer to the ring structure（more stable attraction between them）,the smaller Cr³⁺and Zn²⁺favored in substituting Na1 position;for encapsulation of the largest Cd²⁺,however,the selectivity was disappear.In other words,the selectivity influenced the stability of immobilization of different heavy metals.At the same time,the encapsulation of different heavy metals shrunk the8-member-ring structure at different degree in the following order:Cr>Zn>Cd.The structural shrinkage made it easier for the heavy metal cation to coordinate with extra structured[AlO₄]^-in ring structure,to form R-O coordination,thus the stability of immobilization was improved.The XPS analysis also suggests that the heavy metal cations changed from electrostatically free to R-O and R-OH coordination state when the cations were encapsulated by zeolite P.The selectivity led to the difference of R-O coordination for different heavy metals,indicating that 8-member-ring structure of zeolite P could encapsulate Cr and Zn more readily than Cd.2.The traditional hydrothermal synthesis not only consumes much filling water but also complicates the process.Moreover,the single synthetic zeolite for immobilization of heavy metals is much differed from the actual situation.So the following study modified the synthesis process into the hydrothermal autoclaving by the separation of raw material and filling water,and used Quartz+NaAlO₂ as model soil,which is more similar to the real soil,for hydrothermal synthesis of multi-zeolites and immobilization of heavy metals.The result shows that specific zeolite could be in-situ synthesized by adjusting the addition of ingredients of model soil or the hydrothermal autoclaving conditions.In addition,the improved mass-transfer efficiency made precursor zeolites transformed into more stable analcime and cancrinite much readily.Analcime（ANA,6-member-ring structure of 2.6（?））could be synthesized from model soil with the Al/Si ratio below 0.6 at 180℃for 48 h,cancrinite（CAN,12-member-ring structure of 6.2（?））formed at Al/Si ratio over 1.0and a complex of analcime and cancrinite formed at Al/Si ratio between 0.6 and 1.0.Heavy metals（0.3 mmol/g）polluted model soil could be also converted into analcime,cancrinite and their complex respectively,and the heavy metal could be in-situ immobilized by the formed zeolites during conversion.The curing time,however,was much extended（about 168 hours）due to the retarding effect of heavy metal.EDS and Rietveld refinement analyses reveal that the smaller heavy metal cations(Cr³⁺and Zn²⁺)could partly substitute the only Na（Na1 position）in the6-member-ring channel（2.6（?））of analcime,while the largest heavy metal cation Cd²⁺could hardly substitute the Na（low encapsulation rate）.Cancrinite contains both small6-membered ring and large 12-membered ring（6.2（?））,so that two Na positions（Na1in 6-member-ring channel and Na2 in 8-member-ring channel）can be substituted by heavy metals.The smallest Cr³⁺could substitute Na1 in 6-member-ring channel,but the larger heavy metal cations(Zn²⁺and Cd²⁺)could preferentially substitute Na2 in12-member-ring channel.At the same time,the encapsulation of heavy metals shrunk the framework（unit cell）of both analcime and cancrinite.The structural shrinkage made it easier for different heavy metal cation to coordinate with different ring structure,to form stable R-O coordination.The difference in coordination states（binding energies）of different heavy metals,after the in-situ immobilization by analcime,zeolite P and cancrinite,has been also proved by XPS analysis.The6-member-ring structure is poor at encapsulating large cation of Cd²⁺,and Cd only existed in the form of Cd-OH（poor stability）;the 8-membered ring and 12-membered ring are good at encapsulating Cd²⁺,and Cd existed in the form of Cd-O（better stability）.When the smaller heavy metal cation(Cr³⁺or Zn²⁺)was in-situ immobilized,the encapsulation ability（stability）weakened in the following order:6-memberd ring>8-membered ring>12-membered ring,and the relative content of Cr-O or Zn-O also followed the same order.3.In order to achieve the industrialization readily,this chapter hydrothermally converted real heavy metal polluted soil（with a few mixed water）into analcime and/or cancrinite by autoclaving method without filling water.The result showed that the soil could be converted into analcime when the Al/Si ratio was below 0.6,a complex of analcime and cancrinite formed at Al/Si ratio between 0.6 and 1.0,while only the cancrinite formed over 1.0.In addition,the impurities in the soil can promote the nucleation and growth of zeolites,and accelerate the formation of analcime and cancrinite.Heavy metal polluted（0.3 mmol/g）soil could also be converted into analcime and/or cancrinite hydrothermally,and the result also proved that analcime favored to encapsulate smaller heavy metal cations such as Cr³⁺and Zn²⁺,which formed the stable coordination with frameworkOxygen（Cr-O and Zn-O）in the6-member-ring channel of analcime.However,for the largest heavy metal cation,analcime showed poor encapsulation ability that Cd²⁺mainly formed Cd-OH in analcime.The large 12-member-ring channel of cancrinite was beneficial to encapsulate Cd in the stable form of Cd-O.The large channel contains abundant structural H₂O,which led to the more formation of Cr-OH for Cr³⁺with stronger coordination effect,was negative to stable immobilization of small heavy metal cations.In general,6-member-ring structure performed better encapsulation ability for small heavy metal cations than 12-member-ring structure.Therefore,hydrothermal conversion of heavy metal polluted soil into a complex of zeolites with different multi-member-ring structures（such as analcime and cancrinite）would be more conducive due to the synergistic effect of the structures on encapsulation of heavy metals with different size.4.In order to verify the selectivity and stability of immobilization of heavy metals by different ring structures,TCLP toxic leaching test of the polluted soil before/after hydrothermal conversion was carried out.Attributed to the self-purification ability of soil,the leaching rate/concentration of Cr,Zn and Cd in the polluted soils without hydrothermal treatment were 30%/220 ppm,80%/730 ppm and 80%/1220 ppm respectively.After hydrothermal conversion for 12 h and 168 h,the heavy metals were in-situ immobilized during the formation of zeolites,and the leaching rate of heavy metals significantly reduced to tenth or hundredth of that without hydrothermal treatment,which had the characteristics of long-term stability.Due to the strong encapsulation ability of 6-member-ring channel of analcime for small heavy metals,the leaching rates of Cr and Zn became very low,and decreased with longer treating time when the converted product was analcime,e.g.,leaching rates of Cr and Zn decreased to 1/300 and 1/20 of those before treatment.For Cd²⁺,the leaching rate was only about 1/10 of that before treatment because analcime could hardly encapsulate,but only buried the large Cd²⁺.When the product was cancrinite,the Cd leaching rate could be reduced to as low as 0.7%,about 1/110 of that before treatment,because the 12-member-ring structure was beneficial to the encapsulation of large heavy metal.However,the encapsulation ability of 12-member-ring structure for small heavy metals was poor,prolonging the hydrothermal treating time could not improve the immobilization of Cr or Zn by forming cancrinite（about 1/30 or 1/8 of that before treatment）.The study also disclosed that hydrothermal conversion of heavy metals polluted soil into a complex of zeolites（analcime+cancrinite）not only was very beneficial to encapsulate heavy metal cations in different size,but also could increase the proportion of polluted soil（70 wt%）to improve remediation efficiency.The leaching results also show that the synergistic effect of analcime and cancrinite reduced the leaching rates of all the three heavy metals.The immobilization performance could be further improved by prolonging the treating time,e.g.,the leaching rate of Cr,Zn and Cd was reduced from 2%,33%and 7%（12 hours’treatment）to the lower 1%,13%and 1%（168 hours’treatment）respectively.5.For the concentration of heavy metals in the actual polluted sites is usually much lower than 0.3 mmol/g（the risk screening values for pollution of Cr,Zn and Cd are 610 ppm,1500 ppm and 172 ppm）,the heavy metal polluted soil which were similar to the actual situation(10.7×10^-3 mmol/g:550 ppm for Cr,700 ppm for Zn,1200 ppm for Cd),was zeolitized hydrothermally,and the in-situ immobilization performance by zeolitization was also evaluated in this study.Since the actual soil itself is rich in Al and Si,the polluted soil could be hydrothermally converted into analcime with only a few addition of Na（the proportion of polluted soil was higher than 90 wt%）.For saving treatment cost,the study further tried to replace NaOH with the cheap Na source（NaCl）at high proportion（60%）during hydrothermal conversion.The result suggests that NaCl could provide cubic crystal nucleus for the formation of analcime,and promote the growth of single crystals.During the hydrothermal conversion of the complex of zeolites,the substituted NaCl could also lead to the transformation from cancrinite to sodalite,resulting in the conversion of the complex of analcime+cancrinite into analcime+sodalite.Since the slight changes of heavy metal at low concentration could not be measured by EDS and XPS spectrometer accurately,BCR sequential extraction procedure was used for analysis.The BCR result shows that before hydrothermal treatment,the heavy metals were completely leached from the polluted soil after sequential extraction with no residual fraction,and the exchangeable fractions of Cr,Zn and Cd were high to 10%,75%and 83%respectively.Because of the better encapsulation ability of 6-member-ring structure of analcime for small heavy metal cations,the immobilization stability of Cr and Zn was greatly improved by converting the polluted soil into analcime,e.g.,the residual fractions for Cr and Zn were 35%and 63%（exchangeable fraction of 11%and 5%）respectively.The substitution of NaCl promoted the growth of single crystal of analcime,so that selectivity for encapsulation of small heavy metal cation was further enhanced.As a result,a better stability of Cr than Zn was achieved,e.g.,the residual fraction of 55%（Cr）and 37%（Zn）,and the exchangeable fraction of 1%（Cr）and 11%（Zn）.For the analcime synthesized from Cd polluted soil（without substitution of NaCl）,the poor encapsulation ability of 6-member-ring structure for the largest Cd²⁺has led to the low residual fraction of only 17%.When the polluted soil was hydrothermally converted into the complex of analcime+cancrinite,the synergistic effect of6-membered ring and 12-membered ring significantly improved the stability of all the three heavy metals,and the residual fractions of Cr,Zn and Cd increased to 52%,66%and 46%（exchangeable fractions of 6%,15%and 6%）respectively.Due to the substitution of NaCl,the complex transformed into analcime+sodalite.So that the ability of immobilization of small heavy metal cation was improved,while the immobilization ability of large heavy metal cation decreased.E.g.,the exchangeable fraction of Cr and Zn reached the lowest,2%and 3%respectively,while the exchangeable fraction of Cd increased to 28%.The study confirmed not only the feasibility of rapid and efficient hydrothermal conversion of Si and Al in polluted soil into environmental minerals（zeolites）,but also the capability of in-situ immobilizing/encapsulating heavy metals by multi-member-ring structure of zeolites.The immobilization/encapsulation of heavy metals in different size and charge via hydrothermal zeolitization could significantly improve the stability of immobilization of heavy metal.The results could provide theoretical basis and technical support for rapid hydrothermal remediation of urban heavy metal polluted sites.