Bioavailability of Phosphorus and Sulfur From Pyrolyzed Sewage Sludge.

[NNadir]

A big problem before humanity is the depletion of mined phosphorus, paradoxically coupled with the release of that phosphorus to the ocean and other bodies of water, where it causes eutrophication.

Among our many problems, we need to find ways to close the phosphorus cycle, a non-trivial task.

I won't have much time to discuss a paper across which I just came, but I rather like the work, and feel that it suggests some processes that might help. It's this one: Speciation Evolution of Phosphorus and Sulfur Derived from Sewage Sludge Biochar in Soil: Ageing Effects Hao Sun, Lei Luo, Jiaxiao Wang, Dan Wang, Rixiang Huang, Chenyan Ma, Yong-Guan Zhu, and Zhengang Liu Environmental Science & Technology 2022 56 (10), 6639-6646.

Of course, dried but otherwise untreated sewage sludge is applied to agricultural fields as fertilizer, but this practice leaves something to to be desired.

From the paper's introduction:

Sewage sludge (SS) generally has great potential for land application because of its enrichment of phosphorus (P), sulfur (S) and other nutrient elements induced by human activities. (1?3) Specifically, P and S can amount to 2 and 1% (dry weight) in their contents in SS, respectively, (4,5) which, as reusable resources, are expected to tackle the current dilemma of limited P fertilizers (6,7) and the problem of S deficiency in arable soil. (8,9) On the other hand, land application of SS is frequently discouraged and only 29.3% of SS is disposed via land application in China (10) due to the presence of toxic metals, organic contaminants and pathogenic bacteria. (3,11?13) Therefore, increasingly released SS presents big challenges and opportunities for environmental safety and resource recycling in China. Converting SS into biochar via pyrolysis is proposed as a promising disposal alternative for recycling the organic waste because pyrolysis can greatly decrease the environmental risks of SS by immobilizing heavy metals and decomposing pathogens and organic contaminants. (13?16) Recently, increasing global interest in SS biochar (SSB) (2,17?19) raises a great desire for understanding the speciation of P and S in the biochar and their transformation and fate in soil.

Phosphorus in solid biowaste such as animal manure and SS is gradually transformed into stable Ca-associated compounds like hydroxyapatite during pyrolysis, which can significantly decrease the available fraction of heavy metals in the biowaste through sorption and/or precipitation reactions. (1,4,20,21) Meanwhile, oxidized S in biowaste tends to be reduced and forms stable sulfides with metal cations in the derived biochar. (21,22) Therefore, both P and S play critical roles in immobilizing heavy metals in organic waste during pyrolysis and thus decrease their environmental risks. (21,22) Nevertheless, it has been empirically speculated that the stable Ca-associated phosphate compounds in biochar may increase the availability of P, (20) and sulfides are also liable to be decomposed over time (24) when entering soil environments. In addition, they can affect the solubility and bioavailability of each other in the environment. For example, sulfide can facilitate the release of phosphate bound to iron from SS, (25,26) whereas phosphate can increase available S through competition for sorption sites in SSB-amended soil. (2) It is therefore expected that clarifying the speciation transformation of P and S in SSB induced by soil application will be the key to exploring the immobilization mechanisms of heavy metals and the resource recycling of the nutrients in the environment. (13,21,22) To date, fundamental knowledge on the speciation transformation mechanisms of P and S from SSB in amended soils at the molecular level is still missing (8,20) and thus critically needed...

…The goal of this study was to explore the molecular speciation transformation of P and S derived from SSB as affected by ageing in soil. We hypothesized that the immobilized P and S in SSB stably persist during ageing in soil and thus profoundly affect the availability and reactivity of the nutrients and heavy metals in amended soil. Iron (Fe), as a redox-sensitive element similar to S, can also control the chemical environment of soil and is expected to be closely related to the cycling of P and S in SSB. (12,23) To test these, the speciation of P and S as well as C and Fe from SSB following soil application was investigated based on pot and field experiments.

As chemists of a certain age, when young people did wet chemistry, will know, the sulfides of many heavy metals are relatively insoluble; in fact, the ores of many toxic metals are sulfides. Indeed the ores of many toxic metals are sulfides. Cinnabar, an ore of mercury and galena, an ore of lead, are sulfides. Iron sulfide, found as the mineral pyrite, and iron phosphate are also insoluble.

The authors speculated that on aging, sulfides might liberate the phosphate from iron as a sulfide, and speculated that pyrolyzed sewage sludge might represent a slow release phosphate release agent, fertilizing plants while not producing so much phosphate runoff to produce eutrophication.

Here's some insights to their interesting experimental procedure, at a ton scale:

2. Materials and Methods

2.1. Materials

SS was obtained from a municipal wastewater treatment plant in Wangdu, Hebei Province. SSB was produced from dewatered SS via pyrolysis at 500 °C for 45 min in a 20 ton industrial pyrolysis furnace. A pyrolysis temperature of 500 °C and retention time of 45 min were selected as a compromise of multiple factors including yield, functional group composition, phase transition of biochar structure, immobilization of heavy metals, and energy consumption based on previous studies (21,33) and pre-experiments (Text S1 and Table S1). Field ageing experiments were conducted on uncultivated land in Wangdu, Hebei Province. The soil was a fluvo-aquic soil (Calcaric Cambisol) with a clay-loam texture. Surface soils (0–20 cm), collected from the same location, were used for pot experiments after being air-dried and crushed to less than 2 mm in size.

2.2. Ageing Experiments

Ageing of SSB in soil was conducted through pot and field experiments. Each experiment was performed with three treatments in triplicate: control soil (without SS or SSB), soil with SS, and soil with SSB, at an application rate of 1% (w/w on dried basis). For pot experiments, 1.75 kg soil from each treatment was incubated for 90 days at 75% of its water-holding capacity by periodically adding water. This period could ensure well ageing of SSB to a certain extent. (27,30) For field experiments, a 1-year rotation of summer maize followed by winter wheat was performed in plots from June 10, 2020 to June 10, 2021 to examine long-term ageing effects on the speciation changes of SSB-derived P and S in soil...

The results, as shown in this graphic from the paper, seem to have been promising.



The caption:

Figure 1. Available P and S in soil as affected by SS and SSB amendments, and pot and field incubation, respectively.

SS = sewage sludge. SSB = sewage sludge biochar.

An excerpt of the concluding discussion:

This study explores the distinct speciation evolution of P and S in SSB during pyrolysis and ageing processes and substantiates that speciation of P and S in SSB controls their availability and reactivity in soil environments. The P and S immobilized in SSB through forming stable Ca–P and sulfides can be slowly transformed into relatively available P and S in soil over time. These processes imply that the stable Ca–P in SSB can be a potential source of available P, particularly in the presence of sulfides, while avoiding possible eutrophication by immobilizing excess available P. Most importantly, the immobilized P and S significantly decrease the available fractions of metal cations such as Cd and Cu in SSB (Table S2 and Figure S15) through sorption, precipitation, and sulfidation reactions, and they can continue to play critical roles in maintaining the low risks of heavy metals in the amended soils over a relatively long term through their slow release, for instance, one year, as indicated in this study...

Although heavy toxic metals can be released by the mechanism of oxidation of sulfides, the authors speculate that over time they will become immobilized, but recognize that proof of this might await future work.

I am personally fond of pyrolytic procedures. These require heat, and the only sustainable, reliable way to supply heat of this quality is using nuclear energy, yet another way this much maligned tool might work to save humanity from itself, with the dubious caveat that this would require abandoning our fondness for ignorance.

I trust you will enjoy a pleasant week.