Selective immobilization of Pb(Ⅱ)by biogenic whewellite and its mechanism

The development of bio-adsorbents with highly selective immobilization properties for specific heavy metals is a great challenge,but has important application value.Biogenic whewellite(BW)with high selectivity for Pb(Ⅱ)was synthesized by mineral microbial transformation.The selective immobilization properties and mechanism of BW for Pb(Ⅱ)were analyzed by combining mineral characterization technology and batch adsorption research methods.The results indicated that BW can efficiently and selectively immobilize Pb(Ⅱ)in single or composite heavy metal adsorption solutions,and the immobilized Pb(Ⅱ)is difficult to desorb.BW undergoes monolayer adsorption on Pb(Ⅱ),Qmax≈1073.17 mg/g.The immobilization of Pb(Ⅱ)by BW is a physico-chemical adsorption process with spontaneous heat absorption and an accompanying increase in entropy.In addition,the sequestration of Pb(Ⅱ)by BW remains around 756.99 mg/g even at p H=1.The excellent selective immobilization properties of BW for Pb(Ⅱ)are closely related to its smaller Ksp,electrostatic repulsion effect,organic-inorganic composite structure,acid resistance and the formation of Pb(Ⅱ)oxalate.This study provides beneficial information about the recycling of lead in acidic lead-containing wastewater and composite heavy metal contaminated water bodies.

Biosorption mechanisms involved in immobilization of soil Pb by Bacillus subtilis DBM in a multi-metal-contaminated soil

Mechanisms of soil Pb immobilization by Bacillus subtilis DBM, a bacterial strain isolated from a heavy-metal-contaminated soil, were investigated. Adsorption and desorption experiments with living bacterial cells as well as dead cells revealed that both extracellular adsorption and intracellular accumulation were involved in the Pb2＋removal from the liquid phase. Of the sequestered Pb（II）, 8.5% was held by physical entrapment within the cell wall, 43.3% was held by ion-exchange, 9.7% was complexed with cell surface functional groups or precipitated on the cell surface, and 38.5% was intracellularly accumulated.Complexation of Pb2＋with carboxyl, hydroxyl, carbonyl, amido, and phosphate groups was demonstrated by Fourier transform infrared spectroscopic analysis. Precipitates of Pb5（PO4）3OH, Pb5（PO4）3Cl and Pb10（PO4）6（OH）2that formed on the cell surface during the biosorption process were identified by X-ray diffraction analysis. Transmission electron microscopy–energy dispersive spectroscopic analysis confirmed the presence of the Pb（II）precipitates and that Pb（II） could be sequestered both extracellularly and intracellularly.Incubation with B. subtilis DBM significantly decreased the amount of the weak-acid-soluble Pb fraction in a heavy-metal-contaminated soil, resulting in a reduction in Pb bioavailability, but increased the amount of its organic-matter-bound fraction by 71%. The ability of B.subtilis DBM to reduce the bioavailability of soil Pb makes it potentially useful for bacteria-assisted phytostabilization of multi-heavy-metal-contaminated soil.

The acid dissolution characteristics of cadmium fixed by a novel Ca-Fe-Si composite material

Ca-Fe-Si material(CIS),a novel composite material rich in calcium,iron,manganese and silicon showed marvelous immobilization properties for heavy metal(loid)s in soils.To elucidate the acid stability of Cd fixed by CIS(CIS-Cd)and the underlying immobilizationmechanisms,the acid dissolution characteristics of CIS-Cdwere investigated by using acid titration method and X-ray diffraction(XRD)technique.The results showed that CIS-Cd had distinctive acid buffering capacity in different pH ranges.Based on the titration curve between dissolution rate of CIS-Cd and pH,CIS-Cd can be divided into non acid-stable Cd(9.4%),moderately acid-stable Cd(22.5%)and acid-stable Cd(68.1%).XRD analysis of CIS-Cd at different pH intervals and the correlation curves of dissolution rates of Cd and concomitant elements indicated that non acid-stable Cdwas mainly bound by carbonate,silicate and sulfate(CdCO_(3),Cd_(2)SiO_(4) and CdSO_(4))or co-precipitated with the corresponding calcium salts.Moderately acid-stable Cd was mainly bound by magnesium-aluminum-silicon containing minerals or electrically bound bymanganese iron minerals.Acid-stable Cd remaining undissolved at pH<2.42 included CdFe_(2)O_(4) and ferromanganese minerals strongly bound Cd.It was by multilateral fixation mechanisms that Ca-Fe-Si material possessed marvelous immobilization capability for Cd and strong resilience to environmental acidification as well.The findings implicated that proper combination of calcium-iron-silicon containing minerals could develop novel promising amendments with high efficiency in heavy metal(loid)s immobilization and strong resilience to environmental change.

Co-pyrolysis of sludge and kaolin/zeolite in a rotary kiln:Analysis of stabilizing heavy metals

Pyrolysis is a promising technique used for treating of sewage sludge.However,the application of pyrolysis products is limited due to the presence of heavy metals.In this study,sewage sludge mixed with kaolin/zeolite was pyrolyzed in a rotary kiln,aiming to improve the immobilization of heavy metals in pyrolytic carbon.The total concentrations,speciation distributions,leaching toxicities,and potential ecological risk indices of heavy metals in pyrolysis biochar were explored to examine the effects of kaolin/zeolite and pyrolytic temperature on immobilizing heavy metals.Further,mineral composition and surface morphology of biochar were characterized by X-ray diffraction and scanning electron microscopy to reveal the potential mechanism of immobilizing heavy metals.Increasing pyrolysis temperature facilitated the stabilization of heavy metals in pyrolysis biochar.The proportions of stable heavy metals in biochar obtained at 650℃ were 54.50%(Cu),29.73%(Zn),79.29%(Cd),68.17%(Pb)and 86.70%(Cr).Compared to sewage sludge,the potential contamination risk index of pyrolysis biochar obtained at 650℃ was reduced to 17.01,indicating a low ecological risk.The addition of 7%kaolin/zeolite further reduced the risk index of co-pyrolysis biochar prepared at 650℃ to 10.86/15.28.The characterization of biochar revealed that increase in the pyrolysis temperature and incorporation of additives are conducive to the formation of stable heavy metal-inorganics.This study demonstrates that the formation of stable mineral compounds containing heavy metals is the key to stabilizing heavy metals in pyrolysis biochar.