Speciation Evolution of Phosphorus and Sulfur Derived from Sewage Sludge Biochar in Soil: Ageing Effects

Abstract

Phosphorus (P) and sulfur (S) are usually involved simultaneously in the immobilization of heavy metals in sewage sludge during pyrolysis, and thus their speciation in sewage sludge-derived biochar (SSB) profoundly affects the recycling of the nutrients and the environmental risks of sewage sludge. Here, we investigated the speciation evolution of P and S in SSB induced by ageing processes in soil using X-ray absorption near edge structure spectroscopy. Results showed that Ca-bound compounds like hydroxyapatite dominated the P forms, while over 60% of S existed as reduced inorganic sulfides in the SSB. The stable Ca-associated P species in SSB tended to be transformed gradually into relatively soluble species during ageing in soil. The speciation composition of S in SSB remained almost unaffected when aged in pot soils, whereas about 33.6% of reduced sulfides were transformed into oxidized species after 1-year ageing in field soils. SSB significantly increased the proportion of sulfides and the contents of available P and S in the amended soil but showed relatively weak effects on the speciation distribution of P in the soil because of their similar compositions. These findings provide insights into biogeochemistry of nutrients and behaviors of heavy metals in SSB after its application to the soil environments.

Phosphorus Footprint in the Whole Biowaste–Biochar–Soil–Plant System: Reservation, Replenishment, and Reception

“…After undergoing pyrolysis, a notable trend was observed in the transformation of Fe/Al-adsorbed P into Ca/Mg-associated P, which was consistent with a study from Sun et al, and they also found that Fe−P dominated the P species in sewage sludge, whereas stable Ca−P compounds became the most abundant P species in sewage sludge biochar. 26 In the case of SSBC, more than 80% of P existed in the form of Ca/Mg-associated P. Similarly, the percentage of Ca/Mg-associated P also increased from 70% in raw BD to 85% in BDBC (as shown in Figures 1b and S1). These results suggest that pyrolysis led to the conversion of mobile P species into more stable forms and was retained in biochar, 15 which demonstrates the importance of pyrolysis in improving P retention and stability in biochar.…”

Carbon Sequestration Strategies in Soil Using Biochar: Advances, Challenges, and Opportunities

“…65−67 Interestingly, endogenous minerals can catalyze the decomposition of biomass into small molecules during the carbonization processes and reduce C retention in pyrochar. 30,68 Once entering soil environments, biochar experiences an aging process, 69,70 where it is partially oxidized and releases labile OC. The (released) labile OC as well as the liquid byproduct of hydrochar tends to form aggregates with clay minerals.…”

“…These processes, albeit sounding unbeneficial to C sequestration, are vital for sustaining soil health and productivity , and greatly reshape the turnover and molecular speciation of OC in the amended soils. For instance, biochar increases oxygen-containing functional groups on its recalcitrant surfaces and its labile OC fraction tends to be exhausted during these processes. , Simultaneously, biochar, especially its labile OC fraction, is involved in various reactions with soil components and forms organo-mineral complexes and microaggregates in soil. − Thus, the labile OC and the recalcitrant fraction in biochar are selectively preserved in soil.…”

“…However, the molecular structure is not the sole factor determining the stability of biomass and its derived products in the highly activated ecosystem of soil. Environmental factors and coexisting materials play more important roles. ,, For instance, coexisting minerals can dramatically increase C retention and aromatic structure in pyrochar mainly through physical encapsulation, chemical bonding, and adsorption of CO 2 and CH 4 during carbonization processes and thus significantly enhance its stability. − Interestingly, endogenous minerals can catalyze the decomposition of biomass into small molecules during the carbonization processes and reduce C retention in pyrochar. , Once entering soil environments, biochar experiences an aging process, , where it is partially oxidized and releases labile OC. The (released) labile OC as well as the liquid byproduct of hydrochar tends to form aggregates with clay minerals. ,, The refractory fraction of biochar can also form organometallic complexes with clay minerals , during aging in soil.…”

Remediation of Pb-contaminated soil using biochar-based slow-release P fertilizer and biomonitoring employing bioindicators

“…Biochars alone cannot compete with fertilizer in terms of mineral nutrient contributions. Some authors have found that when biochar ages in the soil, phosphorus is transformed into low bioavailability compounds such as hydroxyapatite (Ca-bound compounds) 10 . Nevertheless, it has also been shown that, over time, these low-solubility compounds can transform into soluble forms of phosphorus 10 .…”