Combined use of biochar and phosphate rocks on phosphorus and heavy metal availability: A meta-analysis

Biochar (BC) and phosphate rocks (PR) are alternative nutrient sources with multiple benefits for sustainable agriculture. The combination of these soil amendments serves two main purposes: to increase soil phosphorus (P) availability and to remediate heavy metal (HM) contamination. However, a further demonstration of the benefits and risks associated with the combined use of BC and PR (BC + PR) is needed, considering the specific characteristics of raw materials, soil types, experimental conditions, and climatic contexts. This meta-analysis is based on data from 28 selected studies, including 581 paired combinations evaluating effects on extraction and fractionation of cadmium (Cd) and lead (Pb), and 290 paired combinations for soil labile and non-labile P. The results reveal that BC, PR, and BC + PR significantly increase soil labile and non-labile P, with BC + PR showing a 150% greater increase compared to BC alone. In tropical regions, substantial increases in P levels were observed with BC, PR, and BC + PR exhibiting increments of 317, 798, and 288%, respectively. In contrast, temperate climate conditions showed lower increases, with BC, PR, and BC + PR indicating 54, 123, and 88% rises in soil P levels. Moreover, BC, PR, and BC + PR effectively reduce the bioavailability of Cd and Pb in soil, with BC + PR demonstrating the highest efficacy in immobilizing Cd. The synergistic effect of BC + PR highlights their potential for Cd remediation. BC + PR effectively reduces the exchangeable fraction of Cd and Pb in soil, leading to their immobilization in more stable forms, such as the residual fraction. This study provides valuable insights into the remediation potential and P management benefits of BC and PR, highlighting their importance for sustainable agriculture and soil remediation practices.

Improved efficiency of Sedum lineare (Crassulaceae) in remediation of arsenic-contaminated soil by phosphate-dissolving strain P-1 in association with phosphate rock

The selection of appropriate plants and growth strategies is a key factor in improving the efficiency and universal applicability of phytoremediation. Sedum lineare grows rapidly and tolerates multiple adversities. The effects of inoculation of Acinetobacter sp. phosphate solubilizing bacteria P-1 and application of phosphate rock (PR) as additives on the remediation efficiency of As-contaminated soil by S. lineare were investigated. Compared with the control, both the single treatment and the combination of inoculation with strain P-1 and application of PR improved the biomass by 30.7-395.5%, chlorophyll content by 48.1-134.8%, total protein content by 12.5-92.4% and total As accumulation by 45.1-177.5%, and reduced the As-induced oxidative damage. Inoculation with strain P-1 increased the activities of superoxide dismutases and catalases of S. lineare under As stress, decreased the accumulation of reactive oxygen species in plant tissues and promoted the accumulation of As in roots. In contrast, simultaneous application of PR decreased As concentration in S. lineare tissues, attenuated As-induced lipid peroxidation and improved As transport to shoots. In addition, the combined application showed the best performance in improving resistance and biomass, which significantly increased root length by 149.1%, shoot length by 33%, fresh weight by 395.5% and total arsenic accumulation by 159.2%, but decreased the malondialdehyde content by 89.1%. Our results indicate that the combined application of strain P-1 and PR with S. lineare is a promising bioremediation strategy to accelerate phytoremediation of As-contaminated soils.

Uranium in phosphate rocks and mineral fertilizers applied to agricultural soils in East Africa

Phosphate rock, pre-concentrated phosphate ore, is the primary raw material for the production of mineral phosphate fertilizer. Phosphate rock is among the fifth most mined materials on earth, and it is also mined and processed to fertilizers in East Africa. Phosphate ore can contain relevant heavy metal impurities such as toxic cadmium and radiotoxic uranium. Prolonged use of phosphate rock powder as a fertilizer and application of mineral fertilizers derived from phosphate rock on agricultural soils can lead to an accumulation of heavy metals that can then pose an environmental risk. This work assesses the uranium concentrations in four major phosphate rocks originating from East Africa and four mineral phosphate fertilizers commonly used in the region. The concentration measurements were performed using energy-dispersive X-ray fluorescence spectrometry. The results showed that the uranium concentration in phosphate rock ranged from as low as 10.7 mg kg^-1 (Mrima Hill deposit, Kenya) to as high as 631.6 mg kg^-1 (Matongo deposit, Burundi), while the concentrations in phosphate fertilizers ranged from 107.9 for an imported fertilizer to 281.0 mg kg^-1 for a local fertilizer produced from Minjingu phosphate rock in Tanzania. In this context, it is noteworthy that the naturally occurring concentration of uranium in the earth crust is between 1.4 and 2.7 mg kg^-1 and uranium mines in Namibia commercially process ores with uranium concentrations as low as 100-400 mg kg^-1. This study thus confirms that East African phosphate rock, and as a result the phosphate fertilizer produced from it can contain relatively high uranium concentrations. Options to recover this uranium are discussed, and it is recommended that public-private partnerships are established that could develop economically competitive technologies to recover uranium during phosphate rock processing at the deposits with the highest uranium concentrations.

Bioimmobilization of lead in phosphate mining wasteland by isolated strain Citrobacter farmeri CFI-01

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.

Bioleaching of rare earths elements from phosphate rock using Acidothiobacillus ferrooxidans

Phosphate rock containing rare earth elements (REEs) is considered one of the most promising potential secondary sources of REEs, as evidenced by large tonnages of phosphate rock mined annually. The bioleaching of REEs from phosphate rock using A. ferrooxidans was done for the first time in this study, and it was found to be greater than abiotic leaching and was more environmentally friendly. The result showed that the total leaching rate of REEs in phosphate rock was 28.46% under the condition of 1% pulp concentration and pH=2, and the leaching rates of four key rare earths, Y, La, Ce, and Nd, were 35.7%, 37.03%, 27.92%, and 32.53%, respectively. The bioleaching process was found to be accomplished by bacterial contact and Fe²⁺ oxidation. The blank control group which contained Fe²⁺ was able to leach some of the rare earths, indicating that the oxidation of Fe²⁺ may affect the leaching of rare earths. X-Ray Diffraction (XRD)analysis showed that the minerals were significantly altered and the intensity of the diffraction peaks of dolomite and apatite decreased significantly after microbial action compared to the blank control, and it was observed that bacteria adhere to the mineral surface and the minerals become smooth and angular after bioleaching by Scanning electron microscope (SEM), indicating that bacteria have a further effect on the rock based on Fe²⁺ oxidation.Finally.Fourier Transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission matrix (3DEEM) fluorescence spectra analysis showed that extracellular polymeric substances (EPS) participate in the bioleaching process.

The boosting of microwave roasting technology on the desulfurization of phosphate rock

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.

Alteration in soil arsenic dynamics and toxicity to sunflower (Helianthus annuus L.) in response to phosphorus in different textured soils

Being analogue to arsenic (As), phosphorus (P) may affect As dynamics in soil and toxicity to plants depending upon many soil and plant factors. Two sets of experiments were conducted to determine the effect of P on As fractionation in soils, its accumulation by plants and subsequent impact on growth, yield and physiological characteristics of sunflower (Helianthus annuus L.). Experimental plan comprised of two As levels (60 and 120 mg As kg^-1 soil), four P (0-5-10-20 g phosphate rock kg^-1 soil) and three textural types (sandy, loamy and clayey) with three replications. Among different As fractions determined, labile, calcium-bound, organic matter-bound and residual As increased while iron-bound and aluminum-bound As decreased with increasing P in all the three textural types. Labile-As percentage increased in the presence of P by 16.9-48.0% at As60 while 36.0-68.1% at As120 in sandy, 19.1-64.0% at As60 while 11.5-52.3% at As120 in loamy, and 21.8-58.2% at As60 while 22.3-70.0% at As120 in clayey soil compared to respective As treatment without P. Arsenic accumulation in plant tissues at both contamination levels declined with P addition as evidenced by lower bioconcentration factor. Phosphorus mitigated the As-induced oxidative stress expressed in term of reduced hydrogen peroxide, malondialdehyde while increased glutathione, and consequently improved the achene yield. Although, P increased As solubility in soil but restricted its translocation to plant, leading to reversal of oxidative damage, and improved sunflower growth and yield in all the three soil textural types, more profound effect at highest P level and in sandy texture.

Activated low-grade phosphate rocks for simultaneously reducing the phosphorus loss and cadmium uptake by rice in paddy soil

Cadmium (Cd) pollution and phosphorus (P) leaching in paddy soils has raised the global concern. In this study, two kinds of the low grade phosphate rocks activated by the sodium lignosulfonate (SL) and humic acid (HA) were fabricated for soil Cd passivation and reduction of the soil P leaching simultaneously. The mechanisms of the Cd adsorption and passivation by the activated phosphate rocks (APRs) were investigated through the batch experiment and the indoor culture test (i.e., incubation and pot experiments) in the Cd-polluted paddy soil. The effects of the APRs on the potted rice growth, uptake of Cd by rice and P loss were also studied. In comparison with the superphosphate treatment, the cumulative P loss from SL- and HA-APRs were reduced by the 65.2% and 65.3%. In terms of the Cd passivation, the Cd adsorbed on the APRs was through the chemical ways (i.e., ligand exchange and the formation of internal complexes). The application of the APRs significantly decreased the soil exchangeable Cd by 48.9%-55.0%, while the Fe/Mn oxides-bound Cd and residual Cd increased significantly by 19.6%-20.3% and 50.7%-69.4%, respectively. Pot experiment also suggested that both the APRs treatments (SL- and HA-APRs) significantly diminished soil Cd accumulation in rice (by 72.7% and 62.8%) coupling with the significantly decreased P leaching. These results provide a sustainable way to explore a novel cost-effective, high-efficient and bi-functional mineral-based soil amendments for environmental remediation.

Novel low-temperature H2S removal technology by developing yellow phosphorus and phosphate rock slurry as absorbent

Recycling hazardous gas of H₂S is one of the most important strategies to promote sustainable development. Herein, a novel method regarding purifying H₂S is proposed by using yellow phosphorus and phosphate rock slurry as absorbent. The H₂SO₄, formed in situ by H₂S conversion, can be devoted to decompose phosphate rock, and the spent absorption slurry was applied as raw material for the production of phosphorus chemical products. According to the characterization analysis, it was found that H₂S was first oxidized to SO₂ via O₂ as well as O₃ induced by P₄. Subsequently, the generated SO₂ dissolved rapidly in water to form H₂SO₄, and then reacted with the main component of phosphate rock, CaMg(CO₃)₂. Most notably, the active substances, such as, O₃, SO₄^•- and OH•, produced in the reaction process, can oxidize H₂S and HS^- to these sulfur products. In addition, trace amounts of Fe³⁺ and Mn²⁺ that were dissolved from phosphate rock displayed a promotional effect on the formation of active substances. Consequently, as high as 85% of H₂S removal efficiency can be obtained even under acidic condition and low temperature. The proposed H₂S purification method offers a promising option for sulfur recovery and H₂S pollution control.

Release behavior of iodine during leaching and calcination of phosphate rock

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.

Implementation of strategies to optimize the co-composting of green waste and food waste in developing countries. A case study: Colombia

Green waste (GW) management is a key issue due to its high production rate and its variety of physical properties and chemical composition. Composting is a promising alternative for GW treatment and valorization. However, the presence of recalcitrant components such as lignin and cellulose increase the processing time. Strategies such as addition of co-substrates and operative modifications have improved the processing time and compost quality. Therefore, in this study, three strategies have been implemented (i) addition of unprocessed food (UF) and processed foods (PF) as co-substrates for GW to improve the nutrients composition of the substrates at the beginning of the process, (ii) addition of phosphate rock (PR) to improve product quality, and (iii) the use of two-stage composting (TSC) to accelerate the degradation. For this purpose, three treatments with the same mixture (48% GW + 21% UF + 18% PF + 13% sawdust (SW)) were conducted: (i) TA (TSC + 15% PR), (ii) TB (traditional composting +15% PR), and (iii) TC (traditional composting). TSC did not show significant differences compared with TC regarding the process and compost quality, while the addition of PR increased the phosphorus content of the product. However, TC produced the compost with the highest quality according to the Colombian legislation for soil amendment.

Phosphorus availability and exchangeable aluminum response to phosphate rock and organic inputs in the Central Highlands of Kenya

Soil acidity and phosphorus deficiency are some of the constraints hampering agricultural production in tropical regions. The prevalence of soil acidity is associated with phosphorus (P) insufficiency and aluminum saturation. We conducted a two-seasons experiment to evaluate soil phosphorus availability and exchangeable aluminum in response to phosphate rock and organic inputs in acidic humic nitisols. The field experiment was installed in Tharaka Nithi County in the Central Highlands of Kenya. The experimental design was a randomized complete block design with treatments replicated thrice. The treatments were: Green manure (Tithonia diversifolia Hemsl.) (60 kg P ha⁻¹), phosphate rock (60 kg P ha⁻¹), goat manure (60 kg P ha⁻¹), Tithonia diversifolia (20 kg P ha⁻¹) combined with phosphate rock (40 kg P ha⁻¹), manure (20 kg P ha⁻¹) combined with phosphate rock (40 kg P ha⁻¹), Triple Super Phosphate combined with Calcium Ammonium Nitrate (TSP + CAN) (60 kg P ha⁻¹) and a control (no input). During the long rains of the 2018 season (LR2018), Tithonia diversifolia + phosphate rock had a significantly higher reduction (67%) of exchangeable aluminum than the sole use of Tithonia diversifolia. Grain yield under TSP + CAN was the highest, followed by the sole organics during the LR2018. Tithonia diversifolia + phosphate rock resulted in a 99% and a 90% increase in NaHCO₃-Pi compared to sole phosphate rock and sole Tithonia diversifolia, respectively. Tithonia diversifolia led to 14% and 62% higher resin-Pi and NaOH-Pi, respectively, compared to manure in the short rains of 2017 (SR2017). The increase in NaOH-Po after the two seasons was statistically significant in sole TSP + CAN. Based on the observed reduced exchangeable aluminum and additional nutrients like Ca, Mg, and K in the soil, sole organic inputs or in combination with phosphate rock treatments are feasible alternatives for sustaining soil phosphorus. Our findings underscore an integrated approach utilizing organic amendments combined with phosphate rock in acidic humic nitisols' phosphorus nutrient management.

Enhanced reduction of lead bioavailability in phosphate mining wasteland soil by a phosphate-solubilizing strain of Pseudomonas sp., LA, coupled with ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.)

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.

Release of fluorine and chlorine during increase of phosphate rock grade by calcination and digestion

A series of experiments for calcination and subsequent digestion to increase the grade of phosphate rock were performed. Fluorine and chlorine released by calcination and digestion were investigated. The forms of both elements were studied by sequential chemical extraction. The grades of raw and calcined phosphate rocks, and their phosphorus concentrates were confirmed according to the value of P₂O₅%. The results showed that fluorine and chlorine in phosphate rock mainly existed in carbonate-bound and residual forms. The grade of phosphate rock can be well increased by calcination and subsequent digestion. By digestion at the optimal condition of 2.5:1 for liquid-solid ratio, 60 °C for digestion temperature and 50 min for digestion time, the phosphorus concentrates digested from the calcined phosphate rock of 1000 °C reached the highest grade (P₂O₅% = 33.24%). By calcination, the released fluorine was mainly HF at a low temperature and SiF₄ at a high temperature, with fluorine release amount of 1.61 × 10⁴-3.82 × 10⁴ g/t at 900-1100 °C. Cl₂, HCl and Cl^- were mainly released at 200-500 °C, less than 800 °C and more than 800 °C, which release amount were 9.40 × 10-2.54 × 10² g/t, 2.10 × 10²-1.53 × 10⁴ g/t and 2.24 × 10²-5.61 × 10² g/t, respectively. By digestion with water for the calcined phosphate rock of 900-1100 °C, the concentrations of fluorine and chlorine in effluent were respectively 77.2-160 mg/L and 7.99-19.6 mg/L. It can be concluded that fluorine and chlorine released by calcination for phosphate rock contribute greatly to atmospheric acid rain and equipment corrosion; by digestion, the discharge of fluorine seriously exceeds the standard and may cause greater pollution to the water body.

Identification of genetic factors controlling phosphorus utilization efficiency in wheat by genome-wide association study with principal component analysis

Despite the economic importance of P utilization efficiency, information on genetic factors underlying this trait remains elusive. To address that, we performed a genome-wide association study in a spring wheat diversity panel ranging from landraces to elite varieties. We evaluated the phenotype variation for P utilization efficiency in controlled conditions and genotype variation using wheat 90 K SNP array. Phenotype variables were transformed into a smaller set of uncorrelated principal components that captured the most important variation data. We identified two significant loci associated with both P utilization efficiency and the 1st principal component on chromosomes 3A and 4A: qPE1-3A and qPE2-4A. Annotation of genes at these loci revealed 53 wheat genes, among which 6 were identified in significantly enriched pathways. The expression pattern of these 6 genes indicated that TraesCS4A02G481800, involved in pyruvate metabolism and TCA cycle, had a significantly higher expression in the P efficient variety under limited P conditions. Further characterization of these loci and candidate genes can help stimulate P utilization efficiency in wheat.

Phosphate-solubilizing Pseudomonas sp., and Serratia sp., co-culture for Allium cepa L. growth promotion

Different genus of bacteria has been reported with the capacity to solubilize phosphorus from phosphate rock (PR). Pseudomonas sp., (A18) and Serratia sp., (C7) isolated from soils at the “Departamento de Boyacá” Colombia, where Allium cepa is cultivated. Bacteria were cultured in MT11B media and evaluated as a bio-fertilizer for A. cepa germination and growth during two months at greenhouse scale. Pseudomonas sp., and Serratia sp., cultured at 30 °C, 48 h in SMRS1 agar modified with PR, (as an inorganic source of phosphorus), presented a phosphate solubilization index (SI) of 2.1 ± 0.2 and 2.0 ± 0.3 mm, respectively. During interaction assays no inhibition halos were observed, demonstrating there was no antagonism between them. In MT11B media growth curve (12 h) demonstrated that co-culture can grow in the presence of PR and glucose concentrations 7.5-fold, lower than in SMRS1 media and brewer's yeast hydrolysate; producing phosphatase enzymes with a volumetric activity of 1.3 ± 0.03 PU at 6 h of culture and 0.8 ± 0.04 PU at 12 h. Moreover, co-culture released soluble phosphorus at a rate of 58.1 ± 0.28 mg L⁻¹ at 8 h and 88.1 ± 0.32 mg L⁻¹ at 12 h. After five days of evaluation it was observed that germination percentage was greater than 90 % of total evaluated seeds, when placing them in contact with the co-culture in a concentration of 1 × 10⁸ CFU mL⁻¹. Furthermore, it was demonstrated that co-culture application (10 mL per experimental unit to complete 160 mL in two months) at 8.0 Log₁₀ CFU mL⁻¹ twice a week for two months increased A. cepa total dry weight (69 ± 13 mg) compared with total dry weight (38 ± 5.0 mg) obtained with the control with water.

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.

Innovation potential along the phosphorus supply chain: A micro and macro perspective on the mining phase

Phosphorus is unique, given its characteristic of being essential for all life on Earth. The element is non-substitutable and finite in the form of highly concentrated phosphate-rock deposits. Thus, humankind should strive to utilize this resource in the most-efficient and sustainable manner. Losses, as well as overlooked opportunities, can be found all along the supply chain in various forms and to distinct extents. Avoiding these by closing the loops all along the supply chain is a key approach for keeping phosphorus flows available for economic use while reducing negative environmental impacts such as eutrophication. Changes to the current, mostly linear approach require multidimensional innovations that address products, processes, structures, and decision-makers along the supply chain as well as societal stakeholders. Our work focuses on the mining phase, covering extraction and beneficiation, whereby we discuss innovation potential in the contexts of i) improving P₂O₅ recovery, ii) utilizing waste, and iii) recovering by-products within the boundaries of sustainable development as generalized strategies on the macro level. Furthermore, we show that there is no "one-size-fits-all" solution to overcome current and future challenges within phosphate-rock mining, as the geological composition and processing of ores differs fundamentally among global deposits. Therefore, we perform, based on previously unpublished primary data, an economic breakdown of production-cost structures covering 85 active phosphate-rock mines and show significant differences between the two main deposit types (i.e., igneous and sedimentary), underground and open-pit mining as well as within each type.

Effective lead immobilization by phosphate rock solubilization mediated by phosphate rock amendment and phosphate solubilizing bacteria

Lead can be immobilized in contaminated soils by phosphate rock (PR) amendment, but its efficiency is generally limited by low solubility of PR. Our study aimed to elucidate whether phosphate solubilizing bacteria (PSB) can promote Pb immobilization through PR solubilization. Results showed that P. ananatis HCR2 and B. thuringiensis GL-1 could effectively solubilize PR by producing citric, glucose, and α-Ketoglutaric acids. In broth assay, phosphate solubilized from PR by PSB rapidly reacted with Pb2+ and formed insoluble lead compounds, as confirmed by scanning electron microscope, energy dispersive X-ray, and X-ray photoelectron spectroscopy. Pot experiment using lettuce (Lactuca sativa L.) and diffusive gradients in thin films (DGT) verified the effectiveness of soil remediation using PR amendment and PSB inoculation, as plant shoot biomass and net photosynthetic rate as well as soil bioavailable phosphate concentration have significantly increased, while the phytoavailability of Pb, Cd, and Zn greatly reduced. This study suggested that PR amendment combined with PSB inoculation could be applied for remediation of agricultural fields contaminated with multiple heavy metals.

Natural radioactivity and radiation hazard assessment of industrial wastes from the coastal phosphate treatment plants of Gabes (Tunisia, Southern Mediterranean Sea)

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.

Phosphate supply security for importing countries: Developments and the current situation

Phosphorus's essentiality for all life on Earth is beyond doubt. As one of the three major macronutrients, it is a crucial building block of modern-day mineral fertilizers and, thus, an essential production factor for global food security. Its almost-exclusive primary sources are phosphate-rock deposits of either sedimentary or igneous origin. The currently known deposits are widely scattered over the globe in large numbers but not necessarily in commercially feasible phosphate-rock quantities. Europe, in particular, possesses negligible considerable reserves; currently, only one Finnish igneous mine is in operation. Thus, phosphate imports are inevitable. Countermeasures like to foster phosphorus recycling from wastewater systems, will reduce rather than substitute imports. Phosphate trade of any form relies on bilateral commercial agreements as it is not traded on commodity exchanges. Therefore, questions regarding supply security and, particularly, the safeness of supply for import-dependent countries, must also address the issues of market concentration, the dynamic reserve-resource situation, and respectively their development over time. We provide a state-of-the-art fundamental analysis all along the supply chain by applying the Hirschman-Herfindahl Index (HHI) to quantify market concentrations as well as reserve-to-production ratios to evaluate developments over recent decades. Thereby, we overcome the unfavorable nature of these static measures. Results suggest medium to highly concentrated markets for the production of phosphate rock, phosphoric acid, and phosphate fertilizers (MAP, DAP) with increasing trends for the overall supply chain. However, these findings should not be interpreted as alarming developments for import-dependent regions, given that the export-market concentrations for phosphate fertilizers have shown significant decreases since the early 1980s.

Oxalic acid activated phosphate rock and bone meal to immobilize Cu and Pb in mine soils

The contamination of soil by copper (Cu) and lead (Pb) is a serious concern because of its high health risk via the food chain. Oxalic acid-activated phosphate rock (APR) and bone meal (BM) were applied to Cu and Pb co-contaminated soil to investigate their efficacy in the immobilization of Cu and Pb. APR and BM were applied into the contaminated soil (158.8 mg/kg total Pb and 573.2 mg/kg Cu) at four levels of dosages (0.1%, 0.5%, 2%, and 4%) and incubated for one year. The results demonstrated that the acid exchangeable Pb fraction in the soil treated with APR and BM decreased compared to the control, while there was no noticeable change in the acid-exchangeable Cu fraction in the soil treated with either APR or BM. Meanwhile, the application of BM and APR increased the fraction of residual Cu and Pb in the polluted soils. Moreover, the addition of either APR or BM at the dose of 4% decreased the concentrations of CaCl₂-extractable Cu and Pb in the amended soil, and the percentages of that reduction in the APR amended soils were 56% and 91% and in BM amended soils were 67% and 64%, respectively. The immobilization of Cu and Pb by APR and BM might be induced by the increased soil pH and soluble P contents in the amended soils. In general, BM is more effective than APR on the immobilization of Cu in polluted soil, while APR had greater efficiency than BM on the immobilization of Pb when the levels of amendments were above 2%.

Arsenic removal by As-hyperaccumulator Pteris vittata from two contaminated soils: A 5-year study

The ability of As-hyperaccumulator Pteris vittata to remove As from two contaminated soils (CCA from an As-treated wood facility and DVA from a cattle-dipping vat) over 5 years was investigated for the first time. The goal was to evaluate P. vittata's ability to continuously remove As during 10 harvests and identify how soil As was affected by P. vittata under P-sufficient (P-fertilizer) and P-limiting (phosphate rock) conditions. Sequential extraction was used to determine changes in metal distribution among different soil fractions. The high frond biomass production occurred on the 9th (62.1-63.9 and 35.6-63.5 g plant^-1) and 10th harvest (58.6-60.7 and 51.9-57.1 g plant^-1) for CCA and DVB soils, though frond As concentration decreased. Soil arsenic removal averaged 7-10% per harvest during the 1-6th harvests and was reduced to 0-3% during the 7-10th harvests for DVA and CCA soils. Arsenic from all fractions, excluding the residual fraction, was affected by plant uptake. The largest reduction occurred in the amorphous fraction of CCA-soil at 64-66% (61.2-61.5 to 20.8-21.8 mg kg^-1) and in the crystalline fraction of DVA-soil at 50-86% (2.18-4.35 to 0.61-1.10 mg kg^-1). Soil As concentrations were reduced by 37-47% from 26.7 to 129 to 15.6-16.8 and 68.9-70.1 mg kg^-1 for the DVA and CCA soils, respectively. Our data indicated that P. vittata efficiently solubilized non-labile As under P-limiting conditions without impacting its As depletion.

Agronomic efficiency of phosphate fertilizers produced by the re-use of a metallurgical acid residue

The production of fertilizers with industrial wastes reduces the environmental impacts of waste disposal and improves environmental sustainability by generating added-value products. Our objective with this study was to evaluate the agronomic performance and potential soil/plant contamination with heavy metals of alternative phosphate (P) fertilizers, obtained from the acidulation of phosphate rocks (PR) by a metallurgical acidic waste. Seven P fertilizers were evaluated: three PR (Araxá, Patos, and Bayóvar), their respective acidulated products (PAPR), and triple superphosphate fertilizer (TSP). A greenhouse trial was carried out to test the agronomic performances of fertilizers in a sequentially cultivated maize-soybean-white oat. The reaction of PR with acid waste was effective to increase their solubility and improve plant yield and P uptake compared to their natural PR. There was a cumulative recovery by plants of 1.4 and 8.1% of added P via PR and PAPR, respectively. No increase in heavy metal (Cd, Pb, Cr, and Ni) availability in soil or accumulation in shoots was observed, indicating that the PAPR were environmentally safe. The usage of acid waste to produce P fertilizers therefore represents a strategic way to employ marginal products for the production of fertilizers.

Enhanced stabilization of Pb, Zn, and Cd in contaminated soils using oxalic acid-activated phosphate rocks

Phosphate amendments, especially phosphate rock (PR), are one of the most commonly used materials to stabilize heavy metals in contaminated soils. However, most of PR reserve consists of low-grade ore, which limits the efficiency of PR for stabilizing heavy metals. This study was to enhance the stabilization of heavy metals through improving the available phosphorous (P) release of PR by oxalic acid activation. Raw PR and activated PR (APR) were characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) surface analysis, and laser diffraction to determine the changes of structure and composition of APR. The stabilization effectiveness of lead (Pb), zinc (Zn), and cadmium (Cd) in soils by APR was investigated through toxicity leaching test and speciation analysis. The results indicated that after treatment by oxalic acid, (1) the crystallinity of the fluorapatite phase of PR transformed into the weddellite phase; (2) the surface area of PR increased by 37%; (3) the particle size of PR became homogenized (20-70 μm); and (4) the available P content in PR increased by 22 times. These changes of physicochemical characteristics of PR induced that APR was more effective to transform soil heavy metals from the non-residual fraction to the residual fraction and enhance the stabilization efficiency of Pb, Zn, and Cd than PR. These results are significant for the future use of low-grade PR to stabilize heavy metals.

[Combination of phosphorus solubilizing and mobilizing fungi with phosphate rocks and volcanic materials to promote plant growth of lettuce (Lactuca sativa L.)]

Arbuscular mycorrhizal fungi (AMF) increase the uptake of soluble phosphates, while phosphorus solubilizing fungi (S) promote solubilization of insoluble phosphates complexes, favoring plant nutrition. Another alternative to maintaining crop productivity is to combine minerals and rocks that provide nutrients and other desirable properties. The aim of this work was to combine AMF and S with pyroclastic materials (ashes and pumices) from Puyehue volcano and phosphate rocks (PR) from Rio Chico Group (Chubut) - to formulate a substrate for the production of potted Lactuca sativa. A mixture of Terrafertil®:ashes was used as substrate. Penicillium thomii was the solubilizing fungus and Rhizophagus intraradices spores (AMF) was the P mobilizer (AEGIS® Irriga). The treatments were: 1) Substrate; 2) Substrate+AMF; 3) Substrate+S; 4) Substrate+AMF+S; 5) Substrate: PR; 6) Substrate: PR+AMF; 7) Substrate: PR+S and 8) Substrate: PR+AMF+S. Three replicates were performed per treatment. All parameters evaluated (total and assimilable P content in substrate, P in plant tissue and plant dry biomass) were significantly higher in plants grown in substrate containing PR and inoculas with S and AMF. This work confirms that the combination of S/AMF with Puyehue volcanic ashes, PR from the Río Chico Group and a commercial substrate promote the growth of L. sativa, thus increasing the added value of national geomaterials.

Arsenic-hyperaccumulator Pteris vittata efficiently solubilized phosphate rock to sustain plant growth and As uptake

Phosphorus (P) is one of the most important nutrients for phytoremediation of arsenic (As)-contaminated soils. In this study, we demonstrated that As-hyperaccumulator Pteris vittata was efficient in acquiring P from insoluble phosphate rock (PR). When supplemented with PR as the sole P source in hydroponic systems, P. vittata accumulated 49% and 28% higher P in the roots and fronds than the -P treatment. In contrast, non-hyperaccumulator Pteris ensiformis was unable to solubilize P from PR. To gain insights into PR solubilization by plants, organic acids in plant root exudates were analyzed by HPLC. The results showed that phytic acid was the predominant (>90%) organic acid in P. vittata root exudates whereas only oxalic acid was detected in P. ensiformis. Moreover, P. vittata secreted more phytic acid in -P and PR treatments. Compared to oxalic acid, phytic acid was more effective in solubilizing PR, suggesting that phytic acid was critical for PR utilization. Besides, secretion of phytic acid by P. vittata was not inhibited by arsenate. Our data indicated that phytic acid played an important role in efficient use of insoluble PR by P. vittata, shedding light on using insoluble PR to enhance phytoremediation of As-contaminated soils.

Arsenic and phosphate rock impacted the abundance and diversity of bacterial arsenic oxidase and reductase genes in rhizosphere of As-hyperaccumulator Pteris vittata

Microbially-mediated arsenic (As) transformation in soils affects As speciation and plant uptake. However, little is known about the impacts of As on bacterial communities and their functional genes in the rhizosphere of As-hyperaccumulator Pteris vittata. In this study, arsenite (AsIII) oxidase genes (aroA-like) and arsenate (AsV) reductase genes (arsC) were amplified from three soils, which were amended with 50mgkg^-1 As and/or 1.5% phosphate rock (PR) and grew P. vittata for 90 d. The aroA-like genes in the rhizosphere were 50 times more abundant than arsC genes, consistent with the dominance of AsV in soils. According to functional gene alignment, most bacteria belonged to α-, β- and γ-Proteobacteria. Moreover, aroA-like genes showed a higher biodiversity than arsC genes based on clone library analysis and could be grouped into nine clusters based on terminal restriction fragment length polymorphism (T-RFLP) analysis. Besides, AsV amendment elevated aroA-like gene diversity, but decreased arsC gene diversity. Redundancy analysis indicated that soil pH, available Ca and P, and AsV concentration were key factors driving diverse compositions in aroA-like gene community. This work identified new opportunities to screen for As-oxidizing and/or -reducing bacteria to aid phytoremediation of As-contaminated soils.

Arsenic-induced plant growth of arsenic-hyperaccumulator Pteris vittata: Impact of arsenic and phosphate rock

Phosphate rock (PR) has been shown to promote plant growth and arsenic (As) uptake by As-hyperaccumulator Pteris vittata (PV). However, little is known about its behaviors in agricultural soils. In this study, impact of 50 mg kg(-1) As and/or 1.5% PR amendment on plant As accumulation and growth was investigated by growing PV for 90 d in three agricultural soils. While As amendment significantly increased plant As uptake and substantially promoted PV growth, the opposite was observed with PR amendment. Arsenic amendment increased plant frond As from 16.9-265 to 961-6017 mg kg(-1),whereas PR amendment lowered frond As to 10.2-216 mg kg(-1). The As-induced plant growth stimulation was 69-71%. While PR amendment increased plant Ca and P uptake, As amendment showed opposite results. The PV biomass was highly correlated with plant As at r = 0.82, but with weak correlations with plant Ca or P at r < 0.30. This study confirmed that 1) As significantly promoted PV growth, probably independent of Ca or P uptake, 2) PR amendment didn't enhance plant growth or As uptake by PV in agricultural soils with adequate available P, and 3) PV effluxed arsenite (AsIII) growing in agricultural soils.

Pteris vittata continuously removed arsenic from non-labile fraction in three contaminated-soils during 3.5 years of phytoextraction

We evaluated the effectiveness of arsenic (As) hyperaccumulator Pteris vittata to continuously remove As from three contaminated-soils containing 26-126mgkg(-1) As over 7 harvests in 3.5 years. Changes in As speciation in soils, amended with P fertilizer (P-soil) or insoluble phosphate rock (PR-soil), were assessed via sequential fractionation. Arsenic in available (soluble+exchangeable), non-labile (bound to amorphous+crystalline Fe/Al oxides), and residual fractions constituted ∼12%, ∼80%, and ∼8% of soil As. Soluble As declined while exchangeable As was unchanged, likely due to replenishment from non-labile As, which accounted for ∼87% of decline in total soil As. Although plant-available As is important, the non-labile As better predicted the frond As concentration in P. vittata, with the correlation being r=0.90 and 0.64 for PR-soils and P-soils. P. vittata removed 44% of soil As from PR-soils compared to 33% from P-soils, suggesting the low-soluble P from PR was more effective than P fertilizer in enhancing As uptake by P. vittata. To facilitate acquisition of P from PR, P. vittata produced larger root biomass to solubilize non-labile As, allowing for more efficient phytoextraction.

Improving Water Use Efficiency of Lettuce (Lactuca sativa L.) Using Phosphorous Fertilizers

A greenhouse pot experiment was conducted to evaluate the effect of phosphorous (P) fertilizers application to an alkaline calcareous soil on the water use efficiency (WUE) of lettuce cultivar “robinson” of iceberg type. Head fresh and dry weights, total water applied and WUE were affected significantly by the P fertilizer type and rate. P fertilizers addition induced a significant enhancement in the WUE and fresh and dry weights of the crop. A local phosphate rock (PR) applied directly was found to be inferior to the other types of P fertilizers (Mono ammonium phosphate (MAP), Single superphosphate (SSP), and Di ammonium phosphate ((DAP)). MAP fertilizer at 375 and 500 kg P₂O₅/ha application rates recorded the highest significant values of head fresh weight and WUE, respectively.

Lead immobilization and bioavailability in microbial and root interface

A range of both soluble and insoluble phosphate (P) compounds have been used to immobilize Pb in solution and soil. However, these compounds have limitations because of low solubility or leaching of P. Phosphate solubilizing bacteria (PSB) can be used to enhance the solubility of insoluble P compounds. The effects of PSB on the immobilization of Pb in the presence of phosphate rock (PR) and subsequent reduction in Pb uptake by Indian mustard (Brassica juncea) in nutrient agar medium and ryegrass (Lolium perenne) in soil under sterile condition were tested. Root colonization of PSB was confirmed by halo formation around the root in the medium containing tricalcium phosphate. Addition of PR in the presence of PSB immobilized Pb in both agar medium and soil, and reduced Pb translocation from root to shoot. Furthermore, shoot Pb concentrations of Indian mustard in agar medium and ryegrass in soil were decreased by 58.1% and 22.8%, respectively, compared to the control. Even though soluble P compound was the most effective in the immobilization of Pb, excess P may cause eutrophication. Therefore, PSB are suggested as a co-amendment to facilitate immobilization of Pb without causing any detrimental effect on the environment.