Bio-Mineralization Of Micro-nano Carbonate And Its Immobilization Mechanisms For Heavy Metals

China's rapid industrialization in the past 40 years has caused serious heavy metals pollution.It is urgent to develop efficient,environmentally friendly,cheap repair materials and appropriate remediation technology.Bio-mineralization of micro-nano carbonates or bio-carbonation of silicate minerals is an important part in surface biogeochemical processes,which profoundly affect the release,migration,transformation and immobilization of heavy metals in the surface environment.It has attracted extensive attention from scholars in the fields of environmental biology,geomicrobiology and biogeochemistry.However,there are still many scientific problems to be solved urgently,such as:the bio-induction conditions and mechanisms of different mineral types micro-nano biogenic calcium carbonate,the immobilization characteristics to heavy metals and their differences in the immobilization capacity to heavy metals compared with abiotic calcium carbonate;the dynamic changes of bio-induced mineralization under heavy metal stress,and the mechanism of microbial regulation during this process,etc.Especially the interaction mechanism and environmental effects between microbe,micro-nano carbonate minerals and heavy metals,are still need to be further studied.In this paper,Bacillus subtilis LB002 was chose as the research strain.Ca Cl₂ was used as the calcium source to induce calcium carbonate formation in LB medium.The changes of p H,HCO₃^-,Ca²⁺concentration,biomass and carbonic anhydrase(CA)activity during calcium carbonate bio-mineralization were detected.Combined with AAS,SEM-EDS,TEM-SAED,soft X-ray imaging technology,XRD,FTIR,TG/DTG-DTA,BET and other technical means to research the bio-mineralization mechanism of calcium carbonate and its non-competitive and competitive adsorption to heavy metals(Cd²⁺,Pb²⁺,Ni²⁺,and Cu²⁺),and exploring the effects of different environmental factors(adsorption dosage,p H,temperature and time)on the adsorption performance of biogenic calcium carbonate.Furthermore,the immobilization capacity of biogenic and abiotic calcium carbonate for Cd²⁺was compared,and its thermodynamic adsorption characteristic was also researched.Finally,we further to research the effect of Cd²⁺on bio-induced mineralization of calcium carbonate and the feasibility of the bacteria weathering silicate minerals(wollastonite,serpentine)to induce the formation of carbonate coupling Cd²⁺immobilization.The main research results are as follows:1.B.subtilis can induce the formation of micro or nano-sized calcium carbonate under the action of its secreted CA and using its own cells as a template.The mineral structure of the calcium carbonate is closely related to the concentration of Ca²⁺.Amorphous calcium carbonate is the main form under low concentration of Ca²⁺(0.2%Ca Cl₂,m/v),while polycrystalline vaterite is the main form under high concentration of Ca²⁺(0.8%Ca Cl₂,m/v).Further studies show that the calcium carbonate induced by B.subtilis is an organic-inorganic mineral complex structure with a good stability.The structural stability of the biogenic amorphous calcium carbonate and vaterite is closely related to the organic matter contained in its minerals,and the organic functional groups in minerals mainly include:-OH,C-H,C=O,NH-CO,COOH,and SH₂,etc.2.Langmuir isotherm adsorption model can well fit the adsorption process of biogenic calcium carbonate and heavy metal ions(Cd²⁺,Pb²⁺,Ni²⁺,Cu²⁺),indicating that the adsorption is mainly monolayer adsorption.Among them,the maximum adsorption capacity of biogenic amorphous calcium carbonate(BACC)for Cd²⁺,Pb²⁺,Ni²⁺,and Cu²⁺is:94.34,416.67,67.11,and 69.93 mg/g,respectively;and the maximum adsorption capacity of biogenic vaterite(BV)for Cd²⁺,Ni²⁺,and Cu²⁺is:172.41,270.27 and 178.57 mg/g,respectively.It can be seen that the adsorption capacity of BV to heavy metals is significantly greater than that of BACC.The metal ions adsorbed by biogenic calcium carbonate are not easy to desorb and have strong ability to resist interference from H⁺ions in aqueous solution,which can maintain strong adsorption capacity even under acidic conditions(Initial p H?3).Further study showed that the structure of biogenic calcium carbonate did not change significantly before and after adsorbing heavy metal ions,which indicating that the adsorption process was dominated by physical adsorption,including mineral surface adsorption and lattice immobilization.3.In the adsorption system of multi-metal ions(Cd²⁺,Ni²⁺,Cu²⁺),the maximum adsorption capacity of BV for Cd²⁺,Ni²⁺,Cu²⁺is 30.30,94.34 and 175.44 mg/g,respectively.It was found that Cu²⁺can significantly inhibit the adsorption of BV for Cd²⁺,Ni²⁺,but the Cd²⁺,Ni²⁺had little effect on the adsorption of Cu²⁺by BV,which indicating that BV has good adsorption selectivity for Cu²⁺.4.The optimum amount of BV for heavy metal ions(Cd²⁺,Ni²⁺,Cu²⁺)is 2.5 g/L.The adsorption capacity of BV for heavy metal ions can be increased significantly with the increase of temperature.The adsorption equilibrium can be reached at 60minutes,and the adsorption processes follow the pseudo-second-order adsorption kinetics model.5.Biogenic calcium carbonate(represented by biogenic vaterite)has greater immobilization capacity for Cd²⁺than abiotic calcium carbonate(represented by chemically synthesized Ca CO₃ and natural limestone),and the adsorption of Cd²⁺by biogenic calcium carbonate is a spontaneous endothermic process(?G<0,?H>0),which is mainly based on physical adsorption.The excellent immobilization capacity of biogenic calcium carbonate is closely related to its unique organic-inorganic composite mesoporous structure,in which the content of organic matter is about 28.49wt%.Compared with abiotic calcium carbonate,the surface of biogenic calcium carbonate is more porous,the crystal size is smaller(7.6 nm),the pore size is better developed,and the pore size mainly distributes around 4.88 nm.Therefore,its BET specific surface area is larger(56.11 m²/g,measured at 300°C for degassing).6.Under the stress of Cd²⁺,B.subtilis can accelerate the mineralization of the micro-nano amorphous calcium carbonate or vaterite,and use these micro-nano biogenic calcium carbonate to adsorb,enwrap and lattice immobilize Cd²⁺in order to reduce the toxicity of Cd²⁺to bacteria,which can significantly enhance the tolerance of the bacteria to Cd²⁺and the enhancement effect of vaterite is more obvious.Further studies showed that the bacteria could also effectively weather wollastonite or serpentine to induce the formation of micro-nano carbonate(Ca CO₃ or Mg CO₃)coupling the immobilization of Cd²⁺,which significantly improving the removal efficiency of B.subtilis to Cd²⁺.These results have great value for the research of the bio-mineralization mechanism and structural analysis of biogenic calcium carbonate.Our findings emphasize the great potential of biogenic calcium carbonate as a medium for the immobilization or passivation of heavy metals in soil or water.It also provides basic data and theoretical basis for the further development and utilization of carbonate bio-mineralization couple heavy metals immobilization technology.