Immobilization Mechanisms And Field Evaluation Of A New Hydroxyapatite Based Binder For Solidification And Stabilization Of Heavy Metal Contaminated Site Soil

Conventional Portland cement?PC?-based binders have several defects,such as high treated soil pH,negatively environmental impacts of PC production,difficulties in applying PC to treat soils with high concentrations of heavy metals,and relatively poor durability performance of treated soils,which have limited their utility in solidification/stabilization?S/S?of heavy metals conamtinated soils.It is necessary to develop new alternative environmental friendly binders,and systematically study their effectiveness,heavy metal immobilization mechanisms,and long-term performances under actual field conditions in treating heavy metal contaminated soils.With financial supports provided by the National High Technology Research and Development Program of China?Grant No.2013AA06A206?,National Natural Science Foundation of China?Grant No.41330641 and 41472258?,and Environmental Protection Scientific Research Project of Jiangsu Province?Grant No.2016031?,the physicochemical properties,strength characteristics,leaching toxicities,heavy metal immobilization mechanisms and microstructure characteristics of lead?Pb?,zinc?Zn?and cadmium?Cd?contaminated site soils stabilized with a new hydroxyapatite based binder SPC are studied by laboratory tests.In addition,the systematical construction processes and technological parameters of in-situ and ex-situ S/S are obtained by field trials.Furthermore,effective diffusion and distribution coefficients of Pb,Zn and Cd,and the long-term effects on water bodies are studied by model tests and numerical simulation.The main finding and conclusions are summarized as follow:?1?Soil pH is increased after SPC treatment,from 6.01?untreated?to?8.20?4%-10%SPC content?,indicating the low alkalinity of stabilized soils.In addition,SPC treatment can significantly decrease the electrical conductivity values and soluble salt contents of contaminated soils.Cementitious products including hydroxyapatite,gypsum,brushite,and calcium-deficient hydroxyapatite significantly reduce the pore volume and also possess high bonding strengths,which results in improvement in the soil strength?unconfined compressive strength up to 439 kPa?.SPC is able to react with Pb,Zn and Cd contaminants to form highly insoluble heavy metal-bearing hydroxyapatites.Additionally,phosphate-based precipitates can also be formed in stabilized soils.SPC remediation hence significantly transforms the exchangeable fractions of Pb,Zn and Cd into more insoluble residual fractions,effectively decrease the leached heavy metla.In addition,improved acid neutralization capacity?ANC?also contributes to the reduced heavy metal leachability of stabilized soils.?2?In-situ or ex-situ S/S using SPC can significantly increase pH values and decrease EC values of contaminated soils.In addition,the leached Pb,Zn and Cd concentrations decrease while penetration resistance increase with increasing SPC content and curing time.In-situ or ex-situ SPC treatment has significantly increased the water extractable TP amount,and the leached TP concentrations from all core samples are well below the surface water regulatory limit.Large percentages of Pb,Zn and Cd are found to be transferred from the exchangeable fraction to residual fraction after SPC remediation,due to the formation of insoluble metal-bearing minerals.These,coupled with the improved ANC,results in the lower metal leachability.?3?The construction procedure of in-situ SPC treatment comprises of the following steps:1)setting up mixing head at the design column position;2)penetration and mixing of soil by the mixing head;3)withdrawing of the mixing head while mixing ground soil with SPC powder using the rotating mixing blades;4)repeat steps 2 and 3 once more;5)completion of the column,and shifting to the next column location;6)repeat the above procedures for all columns until all of them are completed;7)curing of the stabilized soils by covering with waterproof plastic sheets.Key parameters including SPC content,pattern of soil-SPC columns,rate of rotation and powder jet pressure are also presented.?4?The construction procedures for ex-situ SPC treatment are determined during the field testing program.Firstly,the contaminated soil is excavated,transferred and piled in a neighboring open site.Then the contaminated soil is crushed,homogenized,and screened.Subsequently,a certain quantity of the homogenized contaminated soil is sampled and immediately subjected to the modified Proctor test to obtain the compaction curve.After Proctor test,the contaminated soil is tedded and air-dried to the optimum water content.SPC powder is then paved on the surface of the air-dried soil.Subsequently,the thoroughly mixing of SPC binder and air-dried soil is achieved by utilizing a screener-crusher bucket.A pilot test program is performed on the homogeneous stabilized soil to determine the compaction parameter.Subsequently,a 40 cm-thick layer of the 8%SPC stabilized soil is backfilled and spread into the empty pit,then leveled using an excavator.Next,the compaction is conducted with a vibratory roller to achieve the targeted degree of compaction.Once the backfilling and compaction of stabilized soil is completed,the compacted filled soil is cured by covering it with waterproof plastic sheets prior to the field-testing program.Key parameters including compacted layer thickness and number of passes are also presented.?5?Effective diffusion coefficients?D^*?of heavy metals and hydraulic conductivity?k?of stabilized soils are determined by best fit to laboratory measurements.In addition,the effects of stabilized soils on the surrounding water bodies are assessed by numerical simulation.The test results show that Pb,Zn and Cd in 8%SPC stabilized soils have a D^*of 6.7×10^-13,5.0×10^-13 and7.8×10^-12 m²/s respectively.On the other hand,the k_p values of Pb,Zn and Cd are 10400,5200 and1100 mL/g respectively.Morevoer,the 8%SPC stabilized soils show a k value of 6.43×10^-9 m/s.The simulation results indicate that flow velocity of surface water has obvious influences on the risks of heavy metal contamination of surface water with heavy metals transported from contaminated soils,but no effects on the risks of heavy metal contamination of underground water.The leached heavy metal concentrations from stabilized soils to water bodies reach a maximum at 5d of curing,and then decrease with increasing curing time.The heavy metal concentrations in surface and underground water bodies meet the requirements of the Chinese national environmental quality standards for Grade II surface water and Grade II underground water respectively,at a flow velocity of 0.05 and 1.46 m/s.