Phosphorus-based substances such as phosphate esters perform important functions in lubricants as both base stocks and additives. Examples include automotive engine oils, fire-resistant hydraulic fluids and metalworking fluids. Inorganic phosphorus-based substances such as inorganic phosphates also are used in cleaners.

Lubricants and cleaners are used in industrial manufacturing plants. Concern has arisen about the formation of green algae blooms in the western basin of the U.S. Great Lake, Lake Erie, near the city of Toledo, Ohio, during the past 15 years. The most recent algae bloom occurred during the summer of 2017 and was reported to be the third largest on record (the blooms of 2011 and 2015 were bigger).

Contained within the algae blooms were cyanobacteria that can produce a toxin known as microcystis that can cause severe liver damage. One of the most severe outbreaks of microcystis occurred in 2014 when Toledo had to shut down its water system for three days due to cyanobacteria penetrating the city’s water intake pipes.

A recent study found that the algae blooms were directly related to the concentration of phosphorus in the rivers flowing into Lake Erie (1). One source of phosphorus originates from fertilizer used for farming. Roland Cusick, assistant professor in the department of civil & environmental engineering at the University of Illinois at Urbana- Champaign in Urbana, Ill., says, “Phosphorus is originally mined and then converted into phosphoric acid for use as fertilizer in agricultural applications and in industrial applications. Highly soluble fertilizer such as monoand diammonium phosphate can then be washed off fields due to soluble runoff and soil erosion and into waterways.”

One of the most important waterways in the Great Lakes Basin is the Maumee River, which flows Northeast through the farmlands of Indiana and Ohio, through Toledo and into Lake Erie. In September 2017 the Maumee River had turned green as it flowed through Toledo.

As a result of these algae blooms, the U.S. and Canada recently finalized action plans in February to reduce the level of phosphorus flowing into Lake Erie. The U.S. plan to be implemented by EPA can be found at the link in Reference 2.

Concern about end-users limiting or potentially eliminating phosphorus-based substances in their plants has led researchers to determine how to recover phosphorus from effluent waste streams prior to discharge. One such approach is based on precipitating sparingly soluble phosphorus with magnesium or calcium.

Calcium precipitation
Cusick and his colleagues reported that phosphorus byproducts generated during the processing of corn grains into ethanol can be isolated through precipitation using calcium salts. He says, “We worked on this project due to concern about the buildup of phosphorus in the Sangamon River that flows through the center of the U.S. state of Illinois and eventually empties into the Mississippi River basin that flows into the Gulf of Mexico. Two corn processing plants on the Sangamon River were identified as sources of phosphorus.”

Two processes used to extract ethanol from corn grains are wet milling and dry milling. Cusick says, “Steeping the corn grain in wet milling with sulfur dioxide at elevated temperatures for extended periods produces a liquid stream known as Light Steeping Water that can contain between 6,000 and 8,000 milligrams per liter of phosphorus (in comparison to 4-12 milligrams per liter in municipal wastewater). In dry milling, the corn grain is ground up and after fermentation of the starch, a thin stillage fluid is formed that contains between 2,000 and 3,000 milligrams per liter of phosphorus.”

The major source of phosphorus in corn is phytate, inositol hexakisphosphate. The researchers found that treatment of phytate with a calcium salt leads to the precipitation of calcium phytate. For inorganic sources of phosphorus, calcium salts will react to form a calcium phosphate precipitate.

A series of experiments were conducted using calcium and sodium hydroxide to vary the ratio of calcium:phosphorus and pH, respectively, in order to optimize the precipitation process. Cusick says, “The high level of organics in the light steeping fluid isolated from wet milling were very sensitive to changes in pH. This led us to vary the pH from 4.5 to 10.0. We found that a pH above 6.5 and calcium:phosphorus ratios above 1 were needed to maximize phosphorus removal.”

In contrast, dry milling produces more inorganic phosphorus, which is less sensitive to a change in pH. Cusick says, “We found that highest removal of phosphorus from thin stillage fluid to occur at pH values above 8.5 and at calcium:phosphorus rations above 1.5.”

The researchers evaluated the phosphorus-based materials through the use of a scanning electron microscope (SEM). Cusick says, “We identified a heterogenous mixture of particles, and ICP analysis showed other elements (carbon, magnesium, nitrogen and potassium) were present.”

An SEM image showing a micron scale zoom on smaller particulates is found in Figure 3.

Figure 3. A scanning electron microscope image showed phosphorus-based material isolated through calcium precipitation of liquid streams generated during processing of corn grains into ethanol. (Figure courtesy of the University of Illinois at Urbana-Champaign.)

The researchers will undertake several future steps to improve this process. Cusick says, “We will be determining the ratio of organic to inorganic phosphorus using characterization methods such as ₃₁P nuclear magnetic spectroscopy. A model we are developing for maximizing phosphorus precipitation and evaluate system costs will be further calibrated.”

One other objective is to determine how the use of phosphorus recovery units at corn processing plants will impact the economics of waste treatment at POTWs (publicly owned treatment works) treating effluent from biorefineries. Cusick is hopeful that the phosphorus isolated can be recycled for use in fertilizer and other industrial applications. He also thinks that producing animal feeds with low phosphorus content will be of value to animal producers.

Additional information can be found in a recent presentation (3) or by contacting Cusick at rcusick@illinois.edu.
REFERENCES
1. Ho, J. and Michalak, A. (2017), “Phytoplankton blooms in Lake Erie impacted by both long-term and springtime phosphorus loading,” Journal of Great Lakes Research, 43 (3), pp. 221-228.
2. Please click here. 
3. Sharma, N. and Cusick, R. (2018), “Evaluating the potential for phosphorus recovery from side streams at corn ethanol production plants using chemical precipitation,” Presentation given at the American Chemical Society National Meeting & Expo on March 21, 2018, in New Orleans, La.