Composting with chitosan and cat litter composite

The viability of a novel slow-release system for a nitrogen-phosphorus-potassium (NPK) agricultural fertiliser has been tested using atomic absorption spectrometry and shown to have excellent release properties.

The success of an agricultural crop depends primarily on sunlight, water and fertilizer quantity. However, current approaches to adding fertilizer to soil result in huge amounts of waste and potential pollution through run-off.

According to Lan Wu, Mingzhu Liu, and Rui Liang of the Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, at Lanzhou University, and the College of Chemical Engineering, at Northwest Minorities University, in Lanzhou, PR China, slow- or controlled-release fertilizers could improve plant nutrition significantly. Currently, there are three approaches being used to produce such controlled-release fertilisers. These are based on slightly soluble materials, such as urea-formaldehyde, granulated urea compounds for digging in deep, and coated fertilizers that reduce the rate at which the fertiliser dissolves.

Finding an inexpensive coating material that is both non-toxic and biodegradable is a crucial part of research into the latter option. Liu and colleagues have now investigated the biopolymer, chitosan, which is a highly deacetylated derivative of chitin, the main component of the shells of crustaceans, molluscs, insects, and certain fungi and one of the most abundant biopolymers known. Chitin and chitosan have already found widespread application in the biomedical, pharmaceutical, and agricultural fields. "A soluble fertilizer coated by chitosan would be an ideal slow release formulation," Liu and colleagues say.

However, a second material is needed, a superabsorbent polymer, to soak up large amounts of water to allow the fertiliser to be leached out slowly from such a coated particle as well as to improve irrigation in dry regions. Synthetic superabsorbent polymers and those contain sodium ions are inappropriate for agricultural use, so the team has focused on polymer-clay superabsorbent composites based on the highly porous mineral diatomite, commonly used in cat litter, which avoid toxicicity and pollution problems.

Diatomite is a low density sedimentary rock formed from silica microfossils of aquatic unicellular algae (diatoms). It has a three-dimensional network structure as opposed to the layered structure of the more familiar silicates. Voids in diatomite represent between 80 and 90% of its volume. Moreover, it has a surface replete with hydroxyl groups making it very hydrophilic.

The team has now produced a double-coated slow-release NPK composite containing fertilizer and with superabsorbent water-retention properties using this mineral in a composite with polyacrylic acid and with an inner chitosan coating. The benefits of this composite include, not only slow release of NPK fertiliser, but also the aforementioned useful water retention property for improved irrigation efficiency.

Various properties of the material were optimised by fine-tuning the polymer crosslinker, the initiator, degree of neutralization of acrylic acid, initial monomer and diatomite concentration. The team found that the optimal formula had a water absorbency 75 times its own weight at room temperature. They used Fourier transform infrared spectroscopy (FTIR) to characterise the particles. They then used atomic absorption spectrophotometry (AAS) and elemental analysis to demonstrate that it could carry almost 10% each of potassium (by potassium oxide) and phosphorus (revealed by phosphorus(V) oxide) and almost 16% nitrogen.

The study showed the particles to have excellent slow release and water-retention capacity. Indeed, the rough porous surface of the material, dubbed a DSFSW (for double-coated slow-release NPK compound fertilizer with superabsorbent and water-retention), enhances water absorption significantly. The team says that coupled with a lack of toxicity in soil and no harmful environmental effects suggest it could become a useful alternative to conventional fertiliser applications. They demonstrated in slow release experiments that the release ratio of effective nutrient component was no higher than 75% after a month.

"Our results indicate that this composite could be useful in agriculture and horticulture, especially in drought-prone regions," the researchers say.

Kitty litter composite could improve agriculture