Phosphorus Release From Fluorapatite Assisted By Aspergillus Niger And The Application In Cadmium Immobilization

Ecosystem health and sustainability are becoming increasingly threatened by the rising level of heavy metal pollution worldwide.The negative impacts of toxic heavy metals on ecosystems have direct and deleterious effects on food safety and human health.Cadmium(Cd)is one of such toxic heavy metals that causes debilitating diseases in humans and animals,and affects plant growth and developments.Cadmium is non-biodegradable and persists in the environment.Cadmium contamination is wide-spread in many agricultural areas of the world.Therefore,Cd pollution should be tackled urgently.This research addressed the pressing need for an eco-friendly,low cost,and sustainable strategy for the remediation of cadmium-polluted ecosystems.This research investigated the potentials of fungus Aspergillus niger and geological fluorapatite in the remediation of cadmium contaminated water and soil.Aspergillus niger,a phosphate solubilizing fungus,is one of the most common species of the genus Aspergillus.It is ubiquitous in soil.The fungus has been widely applied as an effective additive as biofertilizer in crop production.Phosphate minerals have been proposed to be a promising alternative to the currently available remediation materials because of its abundance,cost-effectiveness,and its important roles in environmental processes.Fluorapatite(FAp)is the most common phosphate mineral form that exists in nature.Fluorapatite is a raw material for the manufacture of P fertilizers,and has been also used directly as P source in agricultural farms.This study involves three key aspects:1.Mineralogy of apatite and cadmium immobilization in aqueous solution by apatiteThis research investigated the application of geological fluorapatite(FAp)in cadmium(Cd)immobilization from aqueous solution.The initial Cd concentrations were set at 100,50,25,and 10 mg L-1.Application of FAp was most efficient in the immobilization of Cd at 100 mg L-1 initial Cd contamination,by reducing the total Cd to 37.7 mg L-1,but less efficient at the medium to lower Cd concentrations.The final Cd concentrations of 37.7,22.3,17.2,and 5.6 mg L-1 were recorded for the amendment with FAp in 100,50,25,and 10 mg L-1 initial Cd concentration,respectively,on the seventh day.The maximum concentrations of released P of 25.8 and 15.2 mg L-1 were recorded in 50 and 25 mg L-1 Cd contamination levels on the second day and third day,respectively.The results showed that FAp is a potential remediation material for the removal of cadmium from water.However,the lower cadmium removal efficiency of FAp observed,especially at the medium to low levels of Cd contamination,implied that a significant amount of mobile Cd was left in the aqueous solution.The combination of FAp with A.niger reduced the Cd in the solutions to final concentrations of 58.5,13.7,3.2,and 0.2 mg L-1 in the solutions,respectively.2.Mechanisms of cadmium immobilization by the combination of A.niger and FAp in solution.Combining two remediation methods needs a careful understanding of the underlying mechanisms of remediation to facilitate the development of the practical protocols for in situ remediation processes.This segment of the research explored the interactions between fungus Aspergillus niger and phosphate mineral(FAp)to understand the underlying mechanisms of Cd immobilization by their combination.This study investigated the fungal biomass,organic acid production,and the formation of minerals as the possible mechanisms of Cd immobilization in aqueous solution.Scanning electron microscopy revealed the proliferation and aggregation of fungal hyphae,and the formation of insoluble Cd-oxalate and Cd-phosphate minerals during the immobilization of Cd.ATR-IR also confirmed the formation of insoluble Cd-oxalate and phosphate complexes.The anti-symmetric carbonyl stretching bands(COO-)specific to the oxalates occurred with peaks located at 1621 and 1538 cm-1 in the combination of the fungus with FAp treatments.The peaks of 1538 and 1311 cm-1,respectively,observed in only Aspergillus niger treatment also confirmed the formation of oxalate complexes.The peaks that confirmed the formation of phosphate minerals(P-O)occurred at 562,and 599 cm-1,respectively,in the combination of the fungus with FAp treatment.Organic acid production was investigated using High Performance Liquid Chromatography(HPLC).This study revealed multiple pathways or mechanisms of Cd immobilization by the combination of the fungus and FAp.3.Cadmium fixation by Aspergillus niger and fluorapatite in the soilThis study evaluates the potential of combining Aspergillus niger and fluorapatite(FAp)in the remediation of cadmium-polluted soils.Parameters such as cadmium(Cd)speciation,microbial abundance,and soil enzyme activities were used to evaluate the effectiveness of the amendments.At 50 mg kg-1 Cd contamination,the concentration of acidsoluble Cd was significantly the lowest at 19.54 mg kg-1 under the combination amendment,than otherwise.The concentrations(mg kg-1)of acid-soluble Cd species under separate applications of FAp and the fungus were 22.05 and 25.34,respectively.At 25 mg kg-1 Cd contamination,the concentration of the acid-soluble Cd was also significantly the lowest at 10.48 mg kg-1 under the combination amendment.The concentrations(mg kg-1)of acidsoluble Cd under applications of FAp and the fungus separately were 11.04 and 11.71,respectively,at 25 mg kg-1 Cd contamination.At 50 mg kg-1 Cd contamination,residual Cd immobilized under the combination amendment was 5.74 mg kg-1,while that immobilized under applications of FAp and the fungus separately were 2.79 mg kg-1 and 1.46 mg kg-1,respectively.At 25 mg kg-1 Cd contamination,the residual Cd immobilized under the combination amendment was 8.42 mg kg-1,while that of applications of FAp and the fungus separately were 4.28 mg kg-1 and 1.27 mg kg-1,respectively.The significantly higher populations of soil microorganisms and the improved enzyme activities observed under the combination amendment also indicate that combining A.niger with FAp effectively reduced Cd bioavailability and toxicity,and restored soil biological activities.This study provided new insights on the potentials and mechanisms of heavy metal immobilization by the combination of soil microorganisms and phosphates.The overall results have shown the great potentials of applying the combination ofAspergillus niger with FAp for the remediation of heavy metal contaminated soil or water.