Concern about algal blooms occurring in the southwest section of the U.S. Great Lake Lake Erie near the city of Toledo has increased due to poor water quality. Cyanobacteria contained within the algal blooms produce a toxin known as microcystis that can cause severe liver damage. A particularly severe outbreak of microcystis in 2014 forced Toledo to shut down its water system for three days.

Currently the citizens of Toledo are frustrated with the lack of action and voted in February (1) to give Lake Erie a Bill of Rights as a way of protecting the lake. One of the potential reasons for the high growth of algae is the presence of phosphorus.

In a past TLT article (2), a recent study indicated that the algae blooms were directly related to the concentration of phosphorus in the rivers flowing into Lake Erie. This factor has led some end-users of phosphorus-based substances including lubricants to limit or eliminate their use. Phosphorus-based additives and base stocks are used widely in lubricants and if restricted would hinder the offerings that the lubricant industry will be able to provide. 

Another source of the phosphorus is from fertilizer used by farmers. In particular, mono and diammonium phosphate have been found to run off from farmers’ fields and enter waterways. A potential approach for recovering phosphorus was discussed in a TLT article (2) and involved using calcium salts to remove phosphate in the form of a water insoluble precipitate. 

Another issue resulting from fertilizer run-off is an increase in nitrogen levels in the form of ammonium salts that also can support the rapid growth of algae. The possibility exists that nitrogen restrictions could lead lubricant end-users to restrict their use. Nitrogen derivatives such as amines are critical additives used in applications such as corrosion inhibitors. Without these additives, lubricant supplies will be more challenged in offering products that meet end-user requirements. 

One approach for minimizing ammonia is to retain it in some natural medium that would prevent its discharge into waterways. Johannes Lehmann, the Liberty Hyde Bailey professor of soil science in the School of Integrative Plant Science at Cornell University in Ithaca, N.Y., says, “Pyrogenic carbon (see Figure 3) has been found to retain nitrogen that is present in the form of ammonia by forming a variety of chemical bonds. Pyrogenic carbon is organic matter or biomass derived from plants that is thermochemically transformed into biochar or charcoal. 


Figure 3. A recent study found that pyrogenic carbon retains ammonia under ambient condition through the formation of covalent bonds between carbon and nitrogen atoms. (Figure courtesy of Cornell University.)

“We typically use the word ‘biochar’ when the material is used for environmental management and specifically when added to soil,” Lehmann adds. “Under natural vegetation fires, approximately 1%-10% of biomass carbon present above ground can end up as pyrogenic carbon. One natural source of pyrogenic carbon is derived from fires taking place in grasslands around the globe. When pyrogenic carbon is made in modern retorts, more than 50% of the carbon can be retained.”

Pyrogenic carbon is a mixture of various organic components. Lehmann says, “Natural materials present in plants including cellulose, lignin, protein and water are converted into a biochar that has a higher concentration of organic carbons as water, hydrogen and oxygen are removed. The carbon components are highly complicated structures including a sizeable presence of aromatic rings and heterocyclic rings that contain nitrogen.”

The appealing aspect of using pyrogenic carbon to retain ammonia is this approach may enable fertilizer to be recycled minimizing its discharge into waterways. Several mechanisms have been proposed but new work has now determined how pyrogenic carbon retains ammonia. 

Covalent bond formation
Lehmann and his fellow researchers are reporting that pyrogenic carbon retains ammonia under ambient conditions and also through the formation of covalent bonds between the carbon and nitrogen atoms. He says, “We know that ammonia gas is very reactive and can form organic derivatives such as amides. Pyrogenic carbon has been found to adsorb some ammonia physically, but our analysis using Fourier transform infrared (FTIR), near-edge X-ray fine structure (NEXAFS) and nuclear magnetic resonance (NMR) spectroscopy showed that a much higher percentage of nitrogen was retained as covalently bound nitrogen than as ammonium salt.”

The degree of ammonia retention increased when pyrogenic carbon was oxidized. Lehmann says, “Pyrogenic carbon remains more persistent in the environment than uncharred organic matter because microbes prefer not to decompose the complex cyclic carbon structures. But microbes oxidize surfaces of pyrogenic carbon. Two other oxidation routes are through photochemical and abiotic means.” 

Lehmann indicated that one of the surprising results was that 10%-20% of the ammonia was retained as part of a cyclic structure (both aromatic and non-aromatic). He says, “Traditionally, such reactions have been described in the past to occur under elevated temperature and pressure conditions, yet we determined through our work that these cyclic structures were formed at room temperature and ambient pressure.”

Pyrogenic carbon in the Earth system has mainly been produced naturally, but Lehmann believes the process also can be done through technologically viable pathways. He says, “There are many ways to convert biomass such as dairy manure through heating in the absence of oxygen into pyrogenic carbon.”

The researchers are working to determine how pyrogenic carbon reacts with ammonia and ammonium ions to form the various compounds detected analytically. Lehmann says, “We do not know what pathway is followed for ammonia to react in a manner with pyrogenic carbon to form heterocyclic nitrogen species.”

The use of natural adsorbers such as pyrogenic carbon has the potential to enable farmers to recycle fertilizer and other nutrients, which could ultimately reduce their operating costs and improve the environment at the same time. Further information on this research can be found in a recent article (3) or by contacting Lehmann at CL273@cornell.edu. 

REFERENCES
1. Henry, T. (2019), “Lake Erie Bill of Rights Gets Approval from Toledo Voters,” Toledo Blade. Available here.
2. Canter, N. (2018), “Recovering Phosphorus from Waste Streams,” TLT, 74 (7), pp. 16-17.
3. Hestrin, R., Rojas, D., Dynes, J., Hook, J., Regier, T., Gillespie, A., Smernik, R. and Lehmann, J. (2019), “Fire-derived Organic Matter Retains Ammonia Through Covalent Bond Formation,” Nature Communications, 10, Article Number: 664.