A potential phosphorus fertilizer to alleviate the coming "phosphorus crisis"-biochar derived from enhanced biological phosphorus removal sludge

Unlocking phosphorus recovery from microalgae biomass: The enhanced transformation and release of phosphorus species

The intertwined challenges of harmful algae blooms and the phosphorus (P) resource crisis have necessitated the recovery of P from algae biomass. For the first time, a co-pyrolysis strategy that incorporates NaHCO3 into the pyrolysis process of chlorella to efficiently recover P in the form of vivianite was proposed. The findings demonstrated that the addition of 20 wt.% NaHCO3 during pyrolysis significantly enhanced P extraction from biochar, increasing the extraction efficiency from 2.8 % to 94.37 %. A complementary array of techniques including chemical extraction, nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), as well as two-dimensional correlation spectroscopy (2D-COS), was employed to elucidate the transformation of hard-to-extract P in chlorella to easy-to-extract P during pyrolysis. It was observed that organophosphorus (OP), pyrophosphate (pyro-P), and polyphosphates (poly-P) reacted with NaHCO3 at 700 °C, undergoing depolymerization and hydrolysis, which led to the formation of orthophosphate (ortho-P) species (e.g., Na3PO4, NaCa(PO4)3, (Fe2(PO4)3), accounting for 98.88 % of the P species in biochar product. High-purity vivianite (∼98.13 %) was subsequently obtained without the need for impurity removal, as indicated by chemical equilibrium simulations, due to the minimal ions and dissolved organic matter (DOM) present in the leaching solution, a consequence of the simple and pure structure of microalgae biomass. The estimated economic profit of this strategy is $1.51 per kilogram of dry chlorella. Additionally, the resulting biochar exhibited a high surface area (518.40 m2/g) and a well-developed pore structure, make it a promising material for adsorption and catalytic applications. This study provides a novel perspective for addressing the P crisis while effectively mitigating harmful algal blooms.

Phosphorus speciation in different sewage sludges and their biochars and its implications for movement of labile phosphate in two soils

It is essential to understand the P dynamics of recycled biomaterials, like biochar derived from sewage sludge, especially with potential application as fertilizers. The objective of this study was to understand how pyrolysis affects the speciation of P in sewage sludge and thereby the effect on labile P pools and mobility of P in soil. The P speciation and lability of two sewage sludges (one biologically treated and one iron-precipitated) and their biochars (pyrolyzed at 400 °C and 600 °C) were determined by liquid state 31P nuclear magnetic resonance spectroscopy, X-ray absorption near edge spectroscopy, and sequential chemical extraction. These biomaterials were applied in a concentrated band to two soils, and P lability was studied in the adjacent soil at varying distances. Speciation techniques showed P was more closely associated with Ca and Fe for the iron-precipitated sludge and its biochars than the biologically treated sludge and its biochars. Instead, the P in the biologically treated biochars was found to be largely (40% or more) in polymeric forms (pyro- or poly-phosphates). The relationship between the speciation and the mobility of P in soil (as assessed by incubating biomaterials in a one-dimensional reaction system) was more evident when incubating the sewage sludges than the respective biochars. Particularly, the biologically treated sludge had a high proportion of labile P (56% water-extractable P), as determined by sequential extraction, and upon incubation, it was also the only material where water-extractable P remained significantly above the control soil level up to 3 mm from the biomaterial layer. After pyrolysis, this lability decreased significantly (up to a 25-fold decrease in water-extractable P), and this was reflected in the immobility of P in the biochars during incubation in the two soils. Differences in speciation between biochars were not reflected in the incubation experiment, as the differences in P release and mobility were not significant.

Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 2: Biochar Characterization and Application in the Remediation of Heavy Metal-Contaminated Soils

The disposal of municipal sewage sludge (MSS) from wastewater treatment plants poses a major environmental challenge due to the presence of inorganic and organic pollutants. Co-pyrolysis, in which MSS is thermally decomposed in combination with biomass feedstocks, has proven to be a promising method to immobilize inorganic pollutants, reduce the content of organic pollutants, reduce the toxicity of biochar and improve biochar's physical and chemical properties. This part of the review systematically examines the effects of various co-substrates on the physical and chemical properties of MSS biochar. This review also addresses the effects of the pyrolysis conditions (temperature and mixing ratio) on the content and stability of the emerging pollutants in biochar. Finally, this review summarizes the results of recent studies to provide an overview of the current status of the application of MSS biochar from pyrolysis and co-pyrolysis for the remediation of HM-contaminated soils. This includes consideration of the soil and heavy metal types, experimental conditions, and the efficiency of HM immobilization. This review provides a comprehensive analysis of the potential of MSS biochar for environmental sustainability and offers insights into future research directions for optimizing biochar applications in soil remediation.

Response of phosphorus fractions transformation and microbial community to carbon-to-phosphorus ratios during sludge composting

Phosphorus (P) is one of the essential nutrient elements for plant growth and development. Sludge compost products can be used as an important source of soil P to solve the shortage of soil P. The difference in the initial carbon-to-phosphorus ratio (C/P) will lead to difference in the bacterial community, which would affect the biological pathway of P conversion in composting. However, few studies have been reported on adjusting the initial C/P of composting to explore P conversion. Therefore, this study investigated the response of P component transformations, bacterial community and P availability to C/P during sludge composting by adjusting initial C/P. The results showed that increasing C/P promoted the mineralization of organic P and significantly increased the content of the labile P. High C/P also increased the relative content of available P, especially when the C/P was at 45 and 60, it reached 60.51% and 60.47%. High C/P caused differences in the community structure, and improved the binding ability of microbial network modules and the competitiveness of microbial communities. Additionally, high C/P strengthened the effect of microbial communities on the transformation of P components. Finally, the study showed that C/P was the main contributor to P content variation (64.7%) and indirectly affected P component conversion by affecting the microbial community. Therefore, adjusting the C/P is crucial to improve the P utilization rate of composting products.

Optimizing directional recovery of high-bioavailable phosphorus from human manure: Molecular-level understanding and assessment of application potential

Phosphorus (P) recovery from human manure (HM) is critical for food production security. For the first time, a one-step hydrothermal carbonation (HTC) treatment of HM was proposed in this study for the targeted high-bioavailable P recovery from P-rich hydrochars (PHCs) for direct soil application. Furthermore, the mechanism for the transformation of P speciation in the derived PHCs was also studied at the molecular level. A high portion of P (80.1∼89.3%) was retained in the solid phase after HTC treatment (120∼240°C) due to high metal contents. The decomposition of organophosphorus (OP) into high-bioavailable orthophosphate (Ortho-P) was accelerated when the HTC temperature was increased, reaching ∼97.1% at 210°C. In addition, due to the high content of Ca (40.45±2.37 g/kg) in HM, the HTC process promoted the conversion of low-bioavailable non-apatite inorganic (NAIP) into high-bioavailable apatite inorganic P (AP). In pot experiments with pea seedling growth, the application of newly obtained PHCs significantly promoted plant growth, including average wet/dry weight and plant height. Producing 1 ton of PHCs (210°C) with the same effective P content as agricultural-type calcium superphosphate could result in a net return of $58.69. More importantly, this pathway for P recovery is predicted to meet ∼38% of the current agricultural demand.

Recent Research on Municipal Sludge as Soil Fertilizer in China: a Review

Due to the annual increase in wastewater treatment in most Chinese cities, a major environmental issue has arisen: safe treatment, disposal, and recycling of municipal sludge. Municipal sludge has a high content of carbon and essential nutrients for plant growth; hence, it has gained interest among researchers as a soil fertilizer. This study discusses the potential usage of municipal sludge as soil fertilizer (indicators include nitrogen (N), phosphorus (P), and trace elements) along with its shortcomings and drawbacks (potentially toxic elements (PTEs), organic matter (OM), pathogens, etc.) as well as reviews the latest reports on the role of municipal sludge in land use. The use of municipal sludge as a soil fertilizer is a sustainable management practice and a single application of sludge does not harm the environment. However, repeated use of sludge may result in the accumulation of harmful chemicals and pathogens that can enter the food chain and endanger human health. Therefore, long-term field studies are needed to develop ways to eliminate these adverse effects and make municipal sludge available for agricultural use.