Industrial phosphate rock (PR) treatment has introduced lead (Pb) contamination into phosphate mining wasteland, causing serious contamination. Although bioremediation is considered an effective method and studies have investigated the bioimmobilization of Pb contamination in phosphate mining wasteland by phosphate-solubilizing bacteria (PSB), the bioimmobilization mechanism remains unclear. In this study, a strain Citrobacter farmeri CFI-01 with phosphate-solubilizing and Pb-tolerant abilities was isolated from a phosphate mining wasteland. Liquid culture experiments showed that the maximum content of soluble phosphate and the percentage amount of Pb immobilized after 14 days were 351.5 mg/L and 98.18%, respectively, with a decrease in pH. Soil experiments showed that CFI-01 had reasonable bioimmobilization ability, and the percentage amount of Pb immobilized was increased by 7.790% and 22.18% in the groups inoculated with CFI-01, respectively, compared with that of the groups not inoculated with CFI-01. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses showed that the immobilization of Pb was also ascribed to changes in the functional groups (e.g., hydroxyl and carboxyl groups) and the formation of lead phosphate sediments. Finally, the results of the metagenomic analysis indicated that changes in the microbial community structure, enrichment of related functional abundances (e.g., metal metabolism, carbohydrate metabolism, and amino acid metabolism functions), and activation of functional genes (e.g., zntA, smtB, cadC, ATOX1, smtA, and ATX1) could help immobilize soil Pb contamination and explore the mechanism of bacterial bioimmobilization in Pb-contaminated soil. This study provides insights for exploring the immobilization mechanism of Pb contamination in phosphate mining wasteland using PSB, which has significance for further research.

Due to ecologically unsustainable mining strategies, there remain large areas of phosphate mining wasteland contaminated with accumulated lead (Pb). In this study, a Pb-resistant phosphate-solubilizing strain of Pseudomonas sp., LA, isolated from phosphate mining wasteland, was coupled with two species of native plants, ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.), for use in enhancing the reduction of bioavailable Pb in soil from a phosphate mining wasteland. The effect of PbCO₃ solubilization by Pseudomonas sp. strain LA was evaluated in solution culture. It was found that strain LA could attain the best solubilization effect on insoluble Pb when the PbCO₃ concentration was 1% (w/v). Pot experiments were carried out to investigate the potential of remediation by ryegrass and sonchus in phosphate mining wastelands with phosphate rock application and phosphate-solubilizing bacteria inoculation. Compared to the control group without strain LA inoculation, the biomass and length of ryegrass and sonchus were markedly increased, available P and Pb in roots increased by 22.2%–325% and 23.3%–368%, respectively, and available P and Pb in above-ground parts increased by 4.44%–388% and 1.67%–303%, respectively, whereas available Pb in soil decreased by 14.1%–27.3%. These results suggest that the combination of strain LA and plants is a bioremediation strategy with considerable potential and could help solve the Pb-contamination problem in phosphate mining wastelands.

The high concentration of fluoride (F) in soils has become a rising concern for its toxicity to microbes, plants, animals and human health. In the present study, the spatial and vertical distribution, health risk assessment and anthropogenic sources of F in farmland soils in an industrial area dominated by phosphate chemical plants were studied. Concentrations of total fluoride (TF) and water soluble fluoride (WSF) in the surface soils decreased with distance within the range of 2500 m at the prevailing downwind of the industrial area. The soil TF and WSF concentrations in 0–40 cm profiles were higher than those in 40–100 cm layers in the industrial area. At the prevailing downwind of the industrial area within 700 m, the hazard quotient values of human exposure to surface soils were higher than 1, indicating that a potential risk may exist for human health in this area. The main exposure pathway for children and adults was oral ingestion and particulate inhalation, respectively. The source apportionment model of soil F was modified based on years’ historical data and experimental data. The results showed that the proportion of anthropogenic sources of soil F was dustfalls (69%) > irrigation water (23%) > air (5%) > chemical fertilizers (3%) in the industrial area. The high F concentration of dustfalls was mainly due to the phosphate rock, phosphogypsum, and surface soils with high F contents from the factories. In order to safeguard human health and alleviate hazards of F to surroundings, the control of pollutants emission from factories was a basic and vital step to reduce F in the soils in industrial areas.

Rhizosphere acidification in leguminous plants can release P from the dissolution of phosphate compounds which can reduce Pb bioavailability to them via the formation of insoluble Pb compounds in their rhizosphere. A soil polluted from Pb-acid batteries effluent (SPBE), having total Pb = 639 mg kg⁻¹, was amended with six different rates (0, 0.5, 1, 2, 4 and 6%) of oxalic acid-activated phosphate rock (OAPR) and their effects on pH, available P and bioavailable Pb concentrations in the rhizosphere and bulk soils of mung bean plant were evaluated. Furthermore, the effects of these variant OAPR rates on Pb concentrations in plant parts, bioaccumulation factor (BAF) and translocation factor (TF) for Pb in grain and traits like productivity, the activities of antioxidant enzymes, and grain biochemistry were investigated. Results revealed that increasing rates of OAPR significantly increased pH values and available P while decreased bioavailable Pb concentrations in the rhizosphere over control. The highest dissolution of P in the rhizosphere was with 4 and 6% OAPR rates. As a result, the formation of insoluble Pb compounds affected on reduced Pb concentrations in shoots, roots, and grain in addition to lower grain BAF and TF values for Pb over control. Likewise, the highest plant productivity, improved grain biochemistry, high Ca and Mg concentrations, least oxidative stress, and enhanced soil alkaline phosphatase activity were found with 4 and 6% OAPR rates. The OAPR 4% rate is suggested for reducing grain Pb concentration, cell oxidative injury, and improving grain biochemistry in mung bean.

This work is a first contribution to the knowledge of natural radionuclides (²²⁶Ra, ²³⁸U, ⁴⁰K, and ²³²Th) activities in phosphate rock (NORM), phosphogypsum, and phosphogypsum foam (TENORM) from the coastal fertilizer plants of Gabes (Southeastern Tunisia) and the assessment of their radiation hazards on human health and the surrounding environment. In the three studied materials, activities were found to be in the range of 35.4 (⁴⁰K)–375.1 (²²⁶Ra), 10.0 (⁴⁰K)–220.2 (²²⁶Ra), and 79.2 (²³²Th)–1168.6 Bq kg⁻¹ (²²⁶Ra), respectively. Considering the studied radionuclides and materials, the corresponding decreasing activity orders were found to be ²²⁶Ra > ²³⁸U > ⁴⁰K > ²³²Th and PGF > PR > PG, respectively. All human health hazard indices exceeded the worldwide recommended safety limits, which show that the workers in Gabes phosphate fertilizer plants as well as the neighboring human community may potentially be exposed to significant radiation, which may cause several diseases and malformations. It is therefore recommended to avoid and/or reduce the potential fertilizer industry radioactive impact in the area.

A green and-easy to operate method, the microwave technology, was developed to promote the desulfurization process of phosphate rock, systematically investigates the strengthening effect of microwave, and uses XRD, BET, SEM, XRF, ICP, and EDS to characterize the reactants. The results show that the main reason for the desulfurization efficiency is improved by microwave heating under microwave conditions, different thermal stress phosphate rock materials lead to the destruction of each microstructure, and a specific surface area increased 40.25% phosphate rock. In addition, after microwave irradiation, the pore size of the phosphate rock at 2–5 nm is significantly increased, and the number of mesopores is significantly increased, thereby increasing the desulfurization efficiency of the phosphate rock. By investigating the effects of temperature, oxygen content, flow rate, and solid-liquid ratio on desulfurization efficiency, the paper concludes that the optimal conditions for desulfurization of phosphate rock after microwave irradiation are C(SO₂) is 2500 mg·m⁻³, temperature is 40 °C, φ(O₂) is 5%, solid-liquid ratio is 3.5 g:200 ml, and flue gas flow is 500 ml·min⁻¹.

Phosphate-solubilizing (PS) microbes are important to improve phosphorus availability and transformation of insoluble phosphate, e.g., rock phosphate (RP). The use of phosphate solubilizing bacteria (PSB) as inoculants have been proposed as an alternative to increase phosphate availability in RP and composting fertilizers. In this study, the effect of compound PSB coinoculation and single-strain inoculation on the transformation of insoluble phosphate were compared in a liquid medium incubation and RP-enriched composting. The goal of this study was to understand the possible mechanisms of insoluble phosphate transformation driven by the interactions of compound PS microbes during composting. The correlations between organic acids production, P-solubilization capacity and bacterial community with PSB inoculation were investigated in the RP-enriched composting by redundancy analysis (RDA) and structural equation models (SEM). Results showed that both single-strain and compound PSB inoculants had a high P-solubilization capacity in medium, but the proportion of Olsen P to total P in composts with inoculating compound PS microbes was 7% higher than that with single strain. PS inoculants could secrete different organic acids and lactic was the most abundant. However, RDA and SEM suggested that oxalic might play an important role on PS activity, inducing RP solubilization by changing pH during composting. Interaction between compound microbes could intensify the acidolysis process for insoluble P transformation compared to the single strain. Our findings help to understand the roles of complex microbial inoculants and regulate P availability of insoluble phosphate for the agricultural purposes.

A series of experiments of column leaching under different pHs (pH 1.8, 3.8, 6.5, and 8.5) and calcination at different temperatures (200–1100 °C) were carried out for evaluation of release behavior of iodine in phosphate rock. The modes of occurrence of iodine in the phosphate rock and its leaching and calcination residues were extracted with sequential chemical extraction. Iodine in solution and solid samples was measured with ion chromatography (IC) and pyrohydrolysis combined ion chromatography (PIC), respectively. Mineralogical compositions of phosphate rock and the leached and calcined residues were determined by XRD (X-ray diffraction) and FT-IR (Fourier infrared spectrum). The results show that iodine in phosphate rock occurred in a descending order of significance, as forms of residual, carbonate bound, ion-exchange, organic bound, Fe-Mn oxide bound, and water soluble. Under pH 1.8, 3.8, 6.5, and 8.5, the release iodine may almost reach the maximum at the leaching time of 65, 93, 90, and 165 h, with leaching rates of 5.28%, 1.24%, 0.550%, and 1.08% and the average iodine concentrations in the leachates of 2300 μg/L, 378 μg/L, 164 μg/L, and 189 μg/L, respectively. The forms of the leached iodine were mostly ion-exchange and carbonate-bound iodine under pH 1.8 and water soluble and ion-exchange iodine under pH 3.8, 6.5, and 8.5. By calcination, the total iodine was released rapidly in 200–300 °C and 700–1000 °C, and almost released completely at 1000 °C, with a leaching rate of 96.6%. The ion-exchange and organic-bound iodine were, respectively, released at 200–1000 °C and at less than 300 °C; the carbonate-bound and residual iodine were mainly released at more than 700 °C. The release iodine in phosphate rock leached by natural water and calcined at a high temperature may lead to the increase of iodine concentration of water body and atmosphere.

We investigated the distributions of phosphate (PO₄-P) and used the oxygen isotope composition of phosphate (δ¹⁸OP) to quantify PO₄-P sources in the waters of Huangbai River. According to the environmental characteristics of Huangbai River basin, the sampling stations in the Huangbai River were divided into three groups: sampling stations in the phosphate mining area, in the outcrop area of phosphate rock, and in the residential/agricultural area. The average PO₄-P concentration was highest (2.34 ± 1.00 μmol/L) in the outcrop area of phosphate ore, intermediate in the residential/agricultural area (1.06 ± 1.21 μmol/L), and lowest in the phosphate mining area (0.58 ± 0.31 μmol/L). The δ¹⁸OP measured in the Huangbai River waters ranged from 6.0 to 20.9‰, with the highest average value in the outcrop area of phosphate rock (average: 14.6‰ ± 3.1‰). The majority of the measured δ¹⁸OP values in the Huangbai River deviated greatly from the expected equilibrium values, indicating that δ¹⁸OP in this area could be used to trace PO₄-P sources. We used two end-member mixing models to quantify the contribution of PO₄-P from different sources. In the phosphate mining area, the average fractions of PO₄-P from phosphate ore and sewage were 49.5% ± 23.8% and 50.5% ± 23.8%, respectively. In the outcrop area of phosphate rock, the average fractions of PO₄-P from phosphate ore and sewage were 60.1% ± 21.7% and 39.9% ± 21.7%, respectively. In the residential/agricultural area, the average fractions of PO4-P from fertilizer and sewage were 49.2% ± 23.2% and 50.8% ± 23.2%, respectively. These results indicate that phosphate mining activities was not an important source for PO₄-P in the waters of Huangbai River. The natural weathering of phosphate rock, fertilization, and domestic sewage contributed more to the high PO₄-P concentrations in the Huangbai River waters.

Synthetic hydroxyapatite (HA) and natural phosphate rock (PR) were applied to heavy metal-contaminated soils from sulfide mine areas in Sardinia and Tuscany (Italy). The application of phosphate amendments to the polluted mine waste soils reduced water-soluble concentrations of Co and Ni by about 99 %. In general, phosphate treatment was slightly more effective in reducing water solubility of Co and Ni in the Sardinian soils than in the Tuscan ones. This result suggests that the mineralogical composition of the mine waste soils may impact the effectiveness of metal immobilization. The formation of complexes of the heavy metals on the surface of phosphate grains and partial dissolution of the amendments and precipitation of heavy metal-containing phosphates are the dominant immobilization mechanisms. Between the phosphate amendments, PR was slightly less effective than HA in immobilizing Co and Ni. This result could be attributed to PR inability to provide soluble phosphate. Although with lower effectiveness, the use of PR to immobilize heavy metals from contaminated soils may reduce the risk of phosphate-induced eutrophication due to the application of amendments with highly soluble phosphate.