Comparison of different phosphorus leaching behaviors of poultry litter and poultry litter char
Date
2011
Authors
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Publisher
University of Delaware
Abstract
In freshwaters, phosphorus is considered the main factor contributing to eutrophication. In US, 60% of the impaired rivers are attributed from eutrophication. Two major phosphorus sources are fertilizers and animal manures. If there is a way to stabilize phosphorus in animal manures, and make the treated animal manures be the substitute of P-fertilizers, this problem can be partially solved. Biochar is the pyrolysis product of biomass. It has drawn increased interests in recent times due to its capacity of serving as long-term carbon storage. We hypothesized that not only biochar can sequester carbon, but it can also stabilize its soluble phosphorus. Thus converting manure into biochar through pyrolysis could reduce the phosphorus leaching potential and represent a potential solution to this problem. This hypothesis was tested by comparing the phosphorus dissolution behaviors of dried poultry litter (PL) pellet and the derived PL char. Dried PL pellet or PL char made from the same batch of pellet was characterized for numerous physical and chemical characteristics. The two materials were further extracted with deionzed water at different solid/leachate ratios for varied time durations. The solutions extractants were then filtered and analyzed for orthophosphate, acid-hydrolyzable phosphorus by colorimetric method and total phosphorus by colorimetric method and ICP-OES. Results showed that the phosphorus in the water extracts of both PL pellet and PL char was almost all orthophosphate. PL pellet rapidly released large amounts of phosphorus and leaching continued for weeks. P released from PL is mostly (about 99.5%) contained in leachate or in particle fractions less than 0.025 μm in diameter. At a 1:100 solid to solution ratio, about 28% of the extractable phosphorus was released to water within the first hour and 50% in the first day. In contrast, PL char released phosphorus rather slowly, giving imparting phosphate concentrations 1-2 orders of magnitude lower than that from dried PL pellet depending on the pH, despite the 98% higher total phosphorus content of the char (on the mass basis). Approximately 80% of the phosphorus released by PL char was in solution, with the remaining phosphorus being particulate (0.025–11 μm). Only 0.3% of the extractable phosphorus was released to water in the first hour. The phosphorus dissolution rate was approximately constant with time. After 12 days, the amount of phosphorus released from PL pellet was 10 times greater than that from PL char. These results suggest that PL char may potentially serve as a slow-releasing and more sustainable source of phosphorus in soil compared with PL pellet. However, the underlying mechanism that controls phosphorus dissolution behavior of PL char requires further investigation. The ion dissolution patterns suggest that the main source of phosphorus might be a magnesium-phosphate mineral in the char, though such a mineral was not identified by X-ray diffraction. Two empirical models have been proposed to describe phosphorus-releasing behaviors of PL pellet and PL char, respectively. But more work is needed to polish the two models and give their parameters meaningful explanations.