Pb Immobilization Research Articles

Use of Brassica juncea and Dactylis glomerata for the phytostabilization of mine soils amended with compost or biochar

Phytostabilization of mine soils contaminated by potentially toxic elements (PTEs) requires plants tolerant to PTE toxicity and to the poor soil physico-chemical characteristics of these areas. A pot experiment was carried out to assess the phytostabilization potential of Brassica juncea and Dactylis glomerata in mine soils amended with compost and biochar. Furthermore, the Environmental Risk of the soils and the effects of the phytostabilization process on the microbiological population size and activity in the soils were also determined. According to the Ecological Risk Index (ERI) the soils studied presented “very high risk” and As, Cd and Pb were the target elements for phytostabilization. Both amendments improved soil conditions (e.g., increasing total-N and total organic-C concentrations) and contributed to PTE (Cd, Pb and Zn) immobilization in the soil. Compost showed a more marked effect on soil microbial biomass and nutrients release in soil, which led to higher B. juncea and D. glomerata biomass in compost treated soils. Biochar treatment showed a positive effect only on D. glomerata growth, despite it provoked strong PTE immobilization in both soils. The addition of both amendments resulted in an overall reduction of PTE concentration in the plants compared to the control treatment. In addition, both plant species showed higher accumulation of PTE in the roots than in the shoots (transfer factor<1) independently of the treatment received. Therefore, they can be considered as good candidates for the phytostabilization of PTE contaminated mine soils in combination with organic amendments like biochar and compost.

Characteristics and mechanisms of Pb(II) sorption onto Fe-rich waste water treatment residue (WTR): A potential sustainable Pb immobilisation technology for soils

Pb contamination of soils is a global problem. This paper discusses the ability of an Fe-rich waste, water treatment residual (WTR), to adsorb Pb(II). This was investigated using batch sorption experiments, X-ray diffraction, electron microprobe microanalysis, PHREEQC modeling and Extended X-ray Absorption Fine Structure (EXAFS) analysis. The WTR is composed of approximately 23 wt. % natural organic matter (NOM), 70 wt. % ferrihydrite and <10 wt. % silicate material. Pb(II) sorption to WTR was dependent on initial Pb(II) load, particle size, time and pH, but not on ionic strength. EXAFS analysis at the Pb LIII-edge confirmed that Pb(II) sorbed to WTR by co-existing bidentate edge-sharing and monodentate or corner-sharing complexes, with 2 O at ∼2.31–2.34 Å, 1 Fe at ∼3.32–3.34 Å, 2 Fe at ∼3.97–3.99 Å and 1 Pb at ∼3.82–3.85 Å. Linear combination showed that the Pb(II)-sorbed spectra were best fit with a ∼0.9 ± 0.1 and 0.1 ± 0.1 contribution from Pb(II)-sorbed ferrihydrite and Pb(II)-sorbed humic acid end members, respectively. Overall, we show that Pb(II) sorbs via strong inner-sphere complexation of Pb(II) to the ferrihydrite component of the WTR, which itself is stable over a wide pH range. Therefore, we suggest that Fe-rich WTR wastes could be used as effective adsorbents in Pb(II)-contaminated soils to help ensure sustainable terrestrial ecosystems.

Metal accumulation and rhizosphere characteristics of Noccaea rotundifolia ssp. cepaeifolia

This work aimed to investigate the metal accumulation characteristics as well as biogeochemical changes in the rhizosphere and root foraging strategies of this plant species. Previous reports suggested that Noccaea rotundifolia ssp. cepaeifolia is a Zn, Cd and Pb hyperaccumulator. We used hydroponic, rhizobox and split-pot experiments for studying metal accumulation and related rhizosphere processes. Although this species accumulated up to 1250 mg Pb kg−1 and 27,000 mg Zn kg−1 in shoots, translocation factors <1 do not meet the hyperaccumulation criteria. Substantial increases in Ca(NO3)2-extractable metals in the N. rotundifolia rhizosphere of a metal-spiked soil can be explained by proton release from N. rotundifolia roots to maintain the charge balance during excessive metal uptake; this was not observed for the non-spiked, moderately contaminated control soil. Specific rhizosphere mechanisms targeting the alleviation of metal toxicity in N. rotundifolia rhizosphere were not detected. Generally, N. rotundifolia had larger total root and shoot mass in soils with heterogeneous distribution of Zn and Pb relative to homogeneous treatments, associated with less root mass placed in metal-enriched patches. However, the avoidance strategy was not reflected by low shoot metal concentrations. Metal accumulation rates and translocation factors do not meet the criteria for hyperaccumulation. Changes of pH and DOC in N. rotundifolia rhizosphere were apparently not involved in targeted immobilisation or detoxification of Pb, Zn and Cd. Avoidance of metal-rich patches in soil is a major tolerance strategy of N. rotundifolia.

Effect of wheat straw derived biochar on immobilization of Cd and Pb in single- and binary-metal contaminated soil

Biochar is a promising adsorption amendment on reducing the bioavailability of heavy metals in contaminated soils. A greenhouse pot experiment was carried out to study the influence of wheat straw biochar (WSB) at different application rates (0%, 0.5%, 1%, and 5% w/w) on pH, electrical conductivity (EC) and toxicity characteristic leaching procedure (TCLP) extractable concentrations of Cd and Pb in single- and binary-metal contaminated soil. Ryegrass (Lolium perenne L.) was used as an indicator plant to evaluate the bioavailability of Cd and Pb. The results showed that the increase in addition of WSB from 0.5% to 5% significantly decreased the TCLP-Cd and Pb concentrations by 9.28–43.59% and 7.45–50.92%, respectively, as compared with control. Pb exhibited more competitive ability than Cd on WSB adsorption sites when coexisted in soil. The application of WSB alleviated the accumulation and uptake of Cd and Pb in the ryegrass shoot and root biomass and facilitated ryegrass growth. Overall, the application of WSB at a high application rate (e.g., 5%) with ryegrass cultivation is an effective method to immobilize Cd and Pb in single- and binary-metal contaminated soil. In the future, it is necessary to determine the long-term field effects of this application.

Immobilization of Lead and Cadmium in Soil Using Biochars Derived from Pig Manure and Suaeda glauca.

Biochar was for the first time produced from Suaeda glauca. The immobilization of Pb and Cd by this biochar and pig manure biochar was examined in two types of soils by diethylene triamine pentaacetic acid (DTPA) extraction. Addition of biochars decreased DTPA extractable Pb and Cd in Fluvo-aquic soil with reduction rates being 11.3%-48.4% and 0.74%-64.9% compared with the control treatment. The pig manure biochar favored the immobilization of Pb and S. glauca biochar favored the immobilization of Cd. Biochars can effectively immobilize heavy metals in Fluvo-aquic soil. However, the addition of biochars increased extractable Pb and Cd in red soil, with pig manure biochars showing greater rates. This is ascribed to that the competition effects of ions released from biochar enhanced the moving of heavy metals from iron and manganese oxides bound form to organic matter bound form, and hence enhanced the mobility and bioavailability of heavy metals.

Iron-modified rice husk hydrochar and its immobilization effect for Pb and Sb in contaminated soil

Cationic and anionic heavy metal contamination sometimes co-exists in soil systems, such as mining areas and shooting ranges, seriously threatens human health and ecological stability. In this study, iron-modified rice husk hydrochar showed commendable ability to immobilize both heavy metal cation (Pb) and anion (Sb) simultaneously in soils. Iron-modified rice husk hydrochar (HC12.5–180) (5%) amendment reduced the bioavailability (EX– and CB–fraction) of Pb and Sb by 25 and 40%, respectively, which were 8 and 5 times higher than that of pristine rice husk hydrochar (HC0–180) (5%) amendment. The cation (Pb) immobilization mainly depends on cation exchange with mineral components (K+, Ca2+, Na+, Mg2+), precipitation with nonmetallic anions (Cl- and SO42-), and complexation. Meanwhile, the iron oxides (FeO, Fe2O3, Fe3O4), formed during hydrothermal process, can be easily combined with anion (Sb) to form geochemically stable minerals. In conclusion, this work offered a practical and cost-effective technology based on the iron modification rice husk hydrochar for the immobilization of both anionic and cationic heavy metal contaminants in soils.

Bamboo-biochar and hydrothermally treated-coal mediated geochemical speciation, transformation and uptake of Cd, Cr, and Pb in a polymetal(iod)s-contaminated mine soil

In this study, polymetal(iod)s-contaminated mining soil from the Huainan coalfield, Anhui, China, was used to investigate the synergistic effects of biochar (BC), raw coal (RC), and hydrothermally treated coal (HTC) on the immobilization, speciation, transformation, and accumulation of Cd, Cr, and Pb in a soil–plant system via geochemical speciation and advanced spectroscopic approaches. The results revealed that the BC-2% and BC–HTC amendments were more effective than the individual RC, and/or HTC amendments to reduce ethylene-diamine-tetraacetic acid (EDTA)-extractable Cd, Cr, and Pb concentrations by elevating soil pH and soil organic carbon content. Soil pH increased by 1.5 and 2.5 units after BC-2% and BC–HTC amendments, respectively, which reduced EDTA-extractable Cd, Cr, and Pb to more stabilized forms. Metal speciation and X-ray photoelectron spectroscopy analyses suggested that the BC–HTC amendment stimulated the transformation of reactive Cd, Cr, and Pb (exchangeable and carbonate-bound) states to less reachable (oxide and residual) states to decrease the toxicity of these heavy metals. Fourier transform infrared spectroscopy and X-ray diffraction analyses suggested that reduction and adsorption by soil colloids may be involved in the mechanism of Cd(II), Cr(VI), and Pb(II) immobilization via hydroxyl, carbonyl, carboxyl, and amide groups in the BC and HTC. Additionally, the BC-2% and BC–HTC amendments reduced Cd and Pb accumulation in maize shoots, which could mainly be ascribed to the reduction of EDTA-extractable heavy metals in the soil and more functional groups in the roots, thus inhibiting metal ion translocation by providing the electrons necessary for immobilization, compared to those in roots grown in the unamended soil. Therefore, the combined application of BC and HTC was more effective than the individual application of these amendments to minimize the leaching, availability, and exchangeable states of Cd, Cr, and Pb in polymetal(iod)s-contaminated mining soil and accumulation in maize.

Green remediation of saline–sodic Pb-factored soil by growing salt-tolerant rice cultivar along with soil applied inorganic amendments

In Pb-factored soils, soil application of appropriate inorganic amendment along with cultivation of salt-tolerant crop species/genotypes for Pb immobilization can be a good choice. Hence, in saline-sodic and non-saline/sodic Pb-factored soils, Pb immobilization was observed as influenced by exogeneously applied inorganic amendments. Salt-tolerant rice cultivar Shaheen basmati was grown in present study. The current pot study consists of two factors: three inorganic amendments (i.e., gypsum, rock phosphate and di-ammonium phosphate) each with their three increasing rates along with uncontaminated and contaminated controls, and two types of Pb-factored soils (non-saline/sodic and saline–sodic). At harvest maturity, crop was reaped and straw, paddy yields and height of plant were noted. The results showed that Pb induced decrease in rice growth and physiological attributes (i.e., transpiration rate, photosynthetic rate, stomatal conductance, chlorophyll a, b and carotenoids) were significantly (p ≤ 0.05) counteracted by exogeneously applied inorganic amendments. Among the tested amendments, the highest gypsum application rate of 9 me Ca 100 g−1 soil was proved the most effective to improve rice growth, yield, physiological functions, and to reduce Pb concentration in rice both in saline-sodic and non-saline/sodic Pb-factored soils.

Eco-friendly remediation for lead-contaminated riverine sediment by sodium lignin sulfonate stabilized nano-chlorapatite

Achieving high immobilization performance to reduce the toxicity and bioavailability of heavy metals is critical for the remediation of heavy metals contaminated sediment. Herein, a novel chlorapatites (SNClAP) with high stability and dispersibility is successfully synthesized using sodium lignin sulfonate (SLS) as dispersant. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) characterization demonstrated that SNClAP possessed an internal layered structure, and exhibited better dispersibility, which was conducive to the immobilization of lead (Pb) in sediment. Besides, SNClAP showed excellent remediation performance towards Pb contaminated sediment, with the highest Pb immobilization efficiency of 80.29%, which was much higher than that of pristine chlorapatite (ClAP) (45.62%). The results of available phosphorus (AP) showed that the better dispersion of chlorapatites could make soluble phosphorus (P) released by chlorapatites more fully combine with Pb to form the extremely stable Pb10(PO4)6Cl2 through precipitation, thus reducing the retention of P in sediment to greatly lower the risk of eutrophication. Therefore, SLS, a by-product of pulping and papermaking industry, was used to synthesize SNClAP, which could not only improve the remediation efficiency of Pb-contaminated sediments, but also greatly promote the recycling of waste, thereby achieving harmony between materials and ecological environment.

Lead immobilization by phosphate in the presence of iron oxides: Adsorption versus precipitation

As a commonly used corrosion inhibitor, phosphate (PO4) has a complicated effect on the fate and transport of lead (Pb) in drinking water systems. While the formation of pyromorphite has been recognized to be the major driving force of the Pb immobilization mechanism, the role of adsorption on iron oxides is still not clear. This study aims to clarify the contributions of adsorption and precipitation to Pb removal in a system containing both iron oxides and PO4. A combination of batch experiments, X-ray absorption spectroscopy, infrared spectroscopy, and electron spectroscopy was employed to distinguish the adsorbed and precipitated Pb species. The results indicated that the adsorption of Pb on iron oxides still occurred even when the solution was supersaturated to pyromorphite (i.e., 5 mg/L P with 0.1–30 mg/L Pb in 0.01 M NaCl solution at neutral pH). In the tap water containing 0.92 mg/L P and 1 mg/L Pb, adsorption on iron oxides contributed more (62–67%) than precipitation (33–38%) in terms of Pb removal. Surprisingly, the pre-formed pyromorphite is transformed to adsorbed species after mixing with iron oxides in water for 24 h. The illustration of this transformation is important to understand the immobilization mechanisms and transport behaviors of Pb in drinking water systems after the utilization of PO4.

Isolation and Characterization of Phosphorus Solubilizing Bacteria With Multiple Phosphorus Sources Utilizing Capability and Their Potential for Lead Immobilization in Soil

Phosphorus solubilizing bacteria (PSB) can promote the level of plant-absorbable phosphorus (P) in agro-ecosystems. However, little attention has been paid to PSB harboring abilities in utilizing multiple phosphorus sources and their potentials for heavy metal immobilization. In this study, we applied the strategy of stepwise acclimation by using Ca3(PO4)2, phytate, FePO4, and AlPO4 as sole P source. We gained 18 PSB possessing abilities of multiple P sources utilization, and these bacteria belonged to eight genera (Acinetobacter, Pseudomonas, Massilia, Bacillus, Arthrobacter, Stenotrophomonas, Ochrobactrum, and Cupriavidus), and clustered to two apparent parts: Gram-positive bacteria and Gram-negative bacteria. The isolate of Acinetobacter pittii gp-1 presented good performance for utilizing Ca3(PO4)2, FePO4, AlPO4, and phytate, with corresponding P solubilizing levels were 250.77, 46.10, 81.99, and 7.91 mg/L PO43–-P, respectively. The PSB A. pittii gp-1 exhibited good performance for solubilizing tricalcium phosphate in soil incubation experiments, with the highest values of water soluble P and available P were 0.80 and 1.64 mg/L, respectively. Additionally, the addition of A. pittii gp-1 could promote the immobilization of lead (Pb), and the highest Pb immobilization efficiency reached 23%. Simultaneously, we found the increases in abundances of both alkaline phosphatase gene (phoD) and β-propeller phytase gene (bpp) in strain gp-1 added soils. Besides, we observed the expression up-regulation of both pyrroloquinoline quinone gene (pqq) and polyphosphate kinases gene (ppk), with the highest relative expression levels of 18.18 and 5.23, respectively. We also found the polyphosphate particles using granule staining. To our knowledge, our findings first suggest that the solubilizing of tricalcium phosphate by phosphorus solubilizing bacterium belonging to Acinetobacter is coupled with the synthesis of polyphosphate. Taken together, A. pittii gp-1 could be a good candidate in improving soil fertility and quality.

Effects of the joint application of phosphate rock, ferric nitrate and plant ash on the immobility of As, Pb and Cd in soils

Phosphate rock (PR) and ferric salts have been frequently used to immobilize heavy metal(loid)s in soils, but in varied efficiencies referring to different metal(loid) pollutants. This study explored the effective application of plant ash (PA) to the previous formula of phosphate rock (PR) and ferric salts (Fe(NO3)3) (PR + Fe3++PA), compared to only PR, on the bioavailability and immobility of multi-metal(loid)s of selected arsenic (As), lead (Pb) and cadmium (Cd) in soils. Results from NaHCO3- extraction and toxicity characteristic leaching procedure (TCLP) implied the increase of the As mobility in soils by 7.0% and 2.6% using PR only, but the significant reduction of the As mobility by 24.2% and 82.4% jointly using PR + Fe3++PA. Meanwhile, the application of either PR alone or PR + Fe3++PA in soil significantly decreased Pb and Cd extracting in diethylene triamine pentacetate acid (DTPA) and TCLP, particularly, the immobilization effect of PR + Fe3++PA was better than that of PR. The leaching column test further confirmed the high durability of PR + Fe3++PA on the immobilization of As and Pb under the continuous acid exposure, but likely slightly increased the mobility of Cd (the accumulated concentration of Cd, 5.88 μg/L) compared to that (3.16 μg/L) in the untreated column (UN-column), which were both much lower than the level V (100 μg/L) of the Chinese National Quality Standard for Surface Water (GB 3838–2002). Therefore, PR + Fe3++PA exhibited the significant enhancement on the immobilization of As, Pb and Cd under simulated acid rain (SAR) leaching.

Adsorption of Cd and Pb in contaminated gleysol by composite treatment of sepiolite, organic manure and lime in field and batch experiments.

Contamination of arable land with trace metals is a global environmental issue which has serious consequences on human health and food security. Present study evaluates the adsorption of cadmium (Cd) and lead (Pb) by using different quantities of composite of sepiolite, organic manure and lime (SOL) at field and laboratory scale (batch experiments). Characterization of SOL by SEM, EDS and FTIR spectroscopy revealed the presence of elemental and functional groups (hydroxyl, C⋯H and -COOH groups) on its surface. The field experiment was performed in a paddy field of gleysol having moderate contamination of Cd and Pb (0.64mgkg-1 and 53.44mgkg-1). Here, different rates of SOL (0.25, 0.5, 1, 1.5 and 2% w/w) were applied by growing low and high Cd accumulator rice cultivars. Application of SOL at 2% w/w showed considerable efficiency to increase soil pH (up to 19%) and to reduce available Cd (42-66%) and Pb (22-55%) as compared to the control. Moreover, its application reduced metal contents in roots, shoots and grains of rice by 31%, 36% and 72% (for Cd) and 41%, 81% and 84% (for Pb), respectively in low accumulator cultivar. Further, the batch sorption experiment was performed to evaluate the adsorption capacity of SOL in a wide range of contamination. Obtained sorption data was better fitted to the Langmuir equation. Our results highlight the strong efficiency of composite treatment for an enhanced in-situ metal immobilization under field and lab conditions. Further, applied treatments greatly reduced the metal contents in rice grains. In a nut shell, application of SOL in a contaminated gleysol should be considered for soil remediation and safe food production.

Heavy metal-immobilizing bacteria increase the biomass and reduce the Cd and Pb uptake by pakchoi (Brassica chinensis L.) in heavy metal-contaminated soil

Microbial immobilization is a novel and environmentally friendly technology that uses microbes to reduce metal availability in soil and accumulation of heavy metals in plants. We used urea agar plates to isolate urease-producing bacteria from the rhizosphere soil of pakchoi in Cd- and Pb-contaminated farmland and investigated their effects on Cd and Pb accumulation in pakchoi and the underlying mechanisms. The results showed that two urease-producing bacteria, Bacillus megaterium N3 and Serratia liquefaciens H12, were identified by screening. They had higher ability to produce urease (57.5mscm-1 min-1 OD600-1 and 76.4mscm-1 min-1 OD600-1, respectively). The two strains allowed for the immobilization of Cd and Pb by extracellular adsorption, bioprecipitation, and increasing the pH (from 6.94 to 7.05-7.09), NH4+ content (69.1%-127%), and NH4+/NO3- ratio (from 1.37 to 1.67-2.11), thereby reducing the DTPA-extractable Cd (35.3%-58.8%) and Pb (37.8%-62.2%) contents in the pakchoi rhizosphere soils and the Cd (76.5%-79.7%) and Pb (76.3%-83.5%) contents in the leaves (edible tissue) of pakchoi. The strains were highly resistant to heavy metal toxicity; produced IAA, siderophores and abscisic acid; and increased the NH4+/NO3- ratio, which might be related to the two strains protectiing pakchoi against the toxic effect of Cd and Pb and increasing pakchoi biomass. Thus, the results were supposed to strain resources and a theoretical basis for the remediation of Cd- and Pb-contaminated farmlands for the safe production of vegetables.

Phosphorus and nitrogen transformation in antibiotic mycelial residue derived hydrochar and activated pyrolyzed samples: Effect on Pb (II) immobilization

In this study, lincomycin residue (LR, a type of antibiotic mycelial residue) derived hydrochar samples (LR-HCs) were obtained from hydrothermal carbonization (HTC), and pyrolysis applied to these LR-HCs to produce activated pyrolyzed samples (LR-APs). Transformation of phosphorus (P) and nitrogen (N) species during HTC and pyrolysis was of primary interest and characterized by several techniques. Nitrogen content of dry LR was calculated by elemental analysis, being 7.91 wt. %, decreasing to 2.51 after HTC and 1.12 wt. % after concesutive HTC and pyrolysis. FT-IR analysis provided evidence for amine groups in LR samples. XPS analysis described N species (Pyridinic-N, Amine-N, Protein-N, Pyrrolic-N, and Quaternary-N) and P species (ortho-P/pyro-P and Ar-P) in LR samples, effectively. Sequential extraction showed that the HTC and pyrolysis changed the proportion of the P species from labile (P-NaHCO3 and P-NaOH) to stable ones (P-residue). Utilization and suitability of as-prepared LR-HCs and LR-APs for heavy metal Pb (II) immobilization show promising results. To help understand immobilization process, kinetic (pseudo-1st-order and pseudo-2nd-order) and isotherm (Freundlich) models were tested and verified. Results confirmed that P and N species were transformed during HTC and pyrolysis and that these processes lead to an advantageous effect on Pb (II) removal from solution.

Preparation and characterization of novel mesoporous chitin blended MoO3-montmorillonite nanocomposite for Cu(II) and Pb(II) immobilization

A novel mesoporous chitin blended MoO3-Montmorillonite nanocomposite was prepared through three-steps synthesis. First, chitin was extracted from prawn shell then MoO3-MMT was prepared, and lastly, chitin was blended with MoO3-MMT. Chitin-MoO3-MMT was applied for the removal of Cu(II) and Pb(II) from wastewater. XRD characterization revealed MoO3 solubility in MMT interlayers, SEM showed a nanocomposite formation with sharp nanorods like-structure and length ranging from 60 to 77.7 nm. FTIR exhibited fundamental changes in the surface functional groups after adsorption. XPS analysis before and after adsorption showed the domination of chemical bonding with N and O. N2 adsorption-desorption isotherm displayed H3-type hysteresis loop and a pore size diameter of 10.67 nm confirming the mesoporous nature. Adsorption efficiency was studied as a function of pH, time, metal concentration and adsorbent mass. Adsorption capacity (Qe) values were 19.03 and 15.92 mg.g−1 for Cu(II) and Pb(II) respectively. The metal surface coverage mapping was 1.87 × 10^19 and 4.34 × 10^18 atoms/m2 for Cu(II) and Pb(II) respectively. Adsorption followed Langmuir isotherm and pseudo-second-order (PSO) kinetics suggesting a monolayer chemisorption domination. Intraparticle diffusion (IPD) model showed a boundary layer control. Thermodynamically, the adsorption was spontaneous and endothermic with activation energies 25.94 and 29.37 kJ.mol−1 for Cu(II) and Pb(II) respectively.

Lead bioimmobilization in contaminated mine soil by Aspergillus niger SANRU

Lead (Pb) contamination in soils is becoming one of the most serious environmental issues in recent years. For this reason, amendment induced immobilization of Pb in contaminated soils has been considered as an eco-friendly and cost-effective technique to minimize soil Pb availability. This study aims at evaluating efficacy of the newly discovered Aspergillus niger strain, SANRU in bioimmobilization of Pb in contaminated mine soil. To conduct bioimmobilization, phosphate rock (PR) at various P/Pb ratios of 0 (T1), 2 (T2), 4 (T3), or 6 (T4) molars was inoculated with the A. niger strain SANRU and subsequently introduced into the soil. The soil sample inoculated with A. niger SANRU and P/Pb molarity ratio of 0 (T1) in this study showed the highest Pb bioimmobilization efficiency, as well as a significant reduction up to 65 % in the unimmobilized Pb concentration and an increase in residual Pb fraction. Our in-depth Fourier transform infrared spectra (FTIR) and X-ray diffraction (XRD) studies revealed that the successful Pb bioimmobilization in this sample was attributed to Pb oxalate formation in the soil. In T2 treated soil with A. niger and P/Pb molarity ratio of 2, the Pb oxalate was precipitated solely as Pb minerals in spite of the presence of available phosphate. In the samples with A. niger and higher amounts of PR (T3 and T4) the Pb bioimmobilization efficiency was significantly lower than T1 but hydroxypyromorphite was formed in these samples. Our data thus propose that A. niger SANRU alone suffice to bioimmobilize Pb in contaminated soil.

Lead immobilization processes in soils subjected to freeze-thaw cycles.

Soil freeze-thaw cycles (FTCs) change the physical and chemical properties of soils; however, information is limited about the consequences for heavy metal sorption and desorption. Lead (Pb) sorption isotherms and successive desorption tests were measured for three soils from North China (Chestnut, Lou and Black), following multiple freeze-thaw cycles (0, 1, 3, 6 and 9 FTCs) of -5°C for 12h and then +5°C for 12h. Lead adsorption dominated the sorption processes for all soils, and sorption capacity increased with additional FTCs. The Freundlich affinity parameter of soils for Pb sorption (i.e. A; Lβ mmol1-β kg-1), was linearly correlated with carbonate content for soils with multiple FTCs. The effects of FTCs on lead adsorption may be more dependent on carbonate and clay contents than organic matter (OM), cation exchange capacity (CEC) and amorphous iron content. Repeated FTCs increased the pH of soil solutions at applied Pb concentrations >1.4mmolL-1, which could facilitate formation of inner-sphere complexes of Pb in studied soils. Cation exchange, a weak association, could occupy specific adsorption sites with increasing Pb doses in soils and it can also be facilitated by FTCs. Our results demonstrate the great potential for increasing Pb immobilization with repeated FTCs, by facilitating the formation of both inner-sphere and outer-sphere complexes. Hence, these findings provide useful information on Pb immobilization in contaminated soils that undergo frequent FTCs and offer an additional insight into predicting Pb behavior in cold and freezing environments like the polar regions.

Towards Understanding the Mechanism of Heavy Metals Immobilization in Biochar Derived from Co-pyrolysis of Sawdust and Sewage Sludge.

Biochar was prepared by mixing sewage sludge with sawdust via a co-pyrolysis with different mixture ratios and temperatures. The results showed that the sawdust addition resulted in a lower yield of biochar with higher C content. The total concentrations of Pb and Cd in biochar were reduced. Besides, pyrolysis can transform the potentially toxic Pb and Cd to stable fractions. However the sawdust addition had slight influence on the chemical forms of Pb and Cd in the biochar. The biochar with 50% sawdust at 600°C exhibited a remarkable reduction of the leachable metal concentrations. The possible transformation mechanisms of Pb and Cd were inferred as the formation of aluminum and silicon-containing minerals. These results provide insights into the influence of sawdust addition on the characteristics of biochar and the possible Pb and Cd immobilization mechanisms during co-pyrolysis process.

Evaluation of biochar pyrolyzed from kitchen waste, corn straw, and peanut hulls on immobilization of Pb and Cd in contaminated soil

Biochar has a wide range of feedstocks, and different feedstocks often resulted in different properties, such as element distribution and heavy metal immobilization performance. In this work, batch experiments were conducted to assess the effectiveness of biochar pyrolyzed from kitchen waste (KWB), corn straw (CSB), and peanut hulls (PHB) on immobilization of Cd and Pb in contaminated soil by planting swamp cabbage (Ipomoea aquatica Forsk.) with a combination of toxicological and physiological tests. The results showed that biochar could all enhance the soil pH, and reduce extractable Pb and Cd in soil by 22.61%–71.01% (KWB), 18.54%–64.35% (CSB), and 3.28%–60.25% (PHB), respectively. The biochar led to a drop in Cd and Pb accumulation in roots, stems, and leaves by 45.43%–97.68%, 59.13%–96.64%, and 63.90%–99.28% at the dosage of 60.00 mg/kg, respectively. The root length and fresh weight of swamp cabbage were promoted, while superoxide dismutase (SOD) and peroxidase (POD) decreased after biochar treatment. The distribution of heavy metal fractions before and after biochar treatment indicated that biochar could transform Cd and Pb into a state of lower bioavailability, thus inhibiting Cd and Pb uptake by swamp cabbage. Biochar with different feedstocks could be ranked by the following order according to immobilization performance: KWB > CSB > PHB.