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Road Salt

December 1, 2014

I've been in Buffalo, New York, once, on business at the regional offices of the Federal Communications Commission. This was in the warmer months, and it included a pleasant excursion to nearby Niagara Falls, the locale of the 1953 Marilyn Monroe film, "Niagara."[1] It was also the setting for the enjoyable, but short-lived, 2004 television series, Wonderfalls.[2] In the past, Niagara Falls was promoted as a honeymoon destination, and it's interesting how Viagra rhymes with Niagara.

Buffalo gained national attention in mid-November, 2014, after being buried under a huge snowfall. Buffalo is no stranger to snow, since its proximity to the Great Lakes leads to considerable quantities of lake-effect snow. Lake-effect snow occurs when eastwardly winds draw moisture from above the lakes and precipitates this as snow. Most of Upstate New York experiences lake-effect snow, as shown in the figure.

Lake-effect snow in New York StateNew York State, showing the regions of lake-effect snow in red. Buffalo is marked with an "X" on this map.

The lakes extend beyond the state boundaries shown.

(Detail of a Wikimedia Commons image, annotated.)

Since humans don't hibernate in winter, people in Buffalo and quite a few other places, just push the snow out of the way and get on with their lives. In Manhattan, and similar places where there are aren't enough places to hold the snow, the snow is scooped into melters and put into the storm sewers where it would have gone eventually. Even when a roadway is nicely plowed, some of the slippery stuff still remains, and that's when chemical snow removal comes into play.

Rock salt, also known as halite, is the traditional method of mitigation for small quantities of snow. It's an inexpensive chemical, and it's relatively benign since we use it to season our food. One taste of my morning oatmeal, cooked by the "old fashioned" method of boiling in a pan on the stove, instantly informs me that I've forgotten to salt the water. Salt has been reviled as being bad for your health, but a little pinch of salt goes a long way in making some foods palatable. Some people add a little salt to their coffee to reduce bitterness, a procedure that actually has a scientific basis.[3-4]

Mixtures of materials will generally have a lower melting/freezing point than the pure substances, as the example of lead-tin solder shows. Lead has a melting point of 327.5 °C, and tin has a melting point of 232 °C, but the common 63% lead/37% tin solder has a melting point of just 183 °C. This same freezing point reduction happens when we add salt to water, and the freezing point can be reduced as low as -18 °C. Salt corrodes both metal and concrete. I wrote about salt corrosion of concrete in a previous article (Salt Corrosion, October 1, 2014).

If there's a need to prevent freezing, or promote thawing, at lower temperatures, other ionic solids, such as magnesium chloride (MgCl2), calcium chloride (CaCl2), or potassium chloride (KCl) can be used. Ionic solids will dissociate into multiple ions in water, and this has an advantage. The van't Hoff factor is the enhancement of the freezing point depression that arises from this dissociation. While non-electrolytes will have a van't Hoff factor of about 1, electrolytes will have a factor roughly equal to the number of ions the electrolyte forms in solution. For this reason, (MgCl2) and calcium chloride (CaCl2) have more "bang-for-the-buck" when used to melt ice.

Snowy street scene in Montreal, Avenue de l'EsplanadeThis photo is a good summary of my childhood in Upstate New York.

A snowy street scene in Montreal.

(Photo by Denis Jacquerye, via Wikimedia Commons.)

Other compounds are used for ice-melting, and airports use various non-flammable glycols, such as propylene glycol, to deice aircraft. Like table salt, propylene glycol is a food additive, so it's generally regarded as safe. Scientists, however, are always looking to advance the status quo, and what better place to conduct research on chemical deicers than Washington State, which experiences weather extremes conducive to icy roadways. In Washington state, about four tons of salt are used per lane mile in winter road maintenance.[5] In Minnesota, nine tons of salt per lane mile are used.[5]

One motivation for such research is that road salt in Washington and other northern states is in short supply this season, and there have been price increases of up to 30%. Another is that heavy salt use is damaging to the environment.[5] The U.S. Environmental Protection Agency reported alarming levels of sodium and chlorine in groundwater for states in the Eastern United States in 2013, and high salt levels can affect potable water supplies.[5]

Winter maintenance of roadways is big business in the US, with $2.3 billion spent each year for removal of highway snow and ice, plus another $5 billion in hidden associated costs.[5] Hidden costs include the environmental impact of sand, salt, and other chemical deicers, and corrosion of the roadways and motor vehicles.[5]

Washington State University is part of a recently established a Center for Environmentally Sustainable Transportation in Cold Climates that's the only center of its type in the United States. The center has received funding of $2.8 million over two years from the US Department of Transportation. Aside from Washington State, the center's participants include the University of Alaska Fairbanks and Montana State University.[5] Says Xianming Shi, associate professor in Civil and Environmental Engineering at Washington State and assistant director of the center,
"We are kind of salt addicted, like with petroleum, as it's been so cheap and convenient for the last 50 years... With a four-lane highway, you have 16 tons of salt per year in that one mile segment... In 50 years, that's about 800 tons of salt in that one mile – and 99 percent of it stays in the environment. It doesn't degrade. It's a scary picture."[5]

Xianming Shi of Washington State UniversityXianming Shi of Washington State University with an industrial-sized mixer he uses to prepare environmentally-friendly deicers.

(Washington State University photograph by Rebecca Phillips.)[5]

Snow and ice control are simple operations, but they require quite a bit of operator judgment. Salt needs to be applied in a proper quantity for the conditions, but the amount is judged visually. Says Shi, "By the time you can see salt on the road, it's way too much and is going into the vegetation and groundwater."[5] One of Shi's research areas is "smart snowplow" technology in which snowplows have integrated sensors that read pavement temperature, salt left from previous applications, the presence of ice, and the amount of road friction.[5]

As an advocate of open source software, I'm happy to see that the Federal Highway Administration has developed the Maintenance Decision Support System, an open source tool that suggests salt application rates based on road and weather conditions.[5] Shi has also been investigating alternative, less corrosive deicers, such as beet and tomato juice, and barley residue from vodka distilleries.[5]

As a complement to this research, Shi is investigating deicer-resistant concrete that also incorporates nanoscale, and larger, particles that produce a surface barrier to prevent bonding with snow and ice. This makes plowing easier, and it decreases the need for salt. Such improvements are useful not just for roadways, but for sidewalks and parking lots.[5] Shi presented his approaches to winter roadway maintenance at the American Public Works Association Western Snow and Ice Conference in September.[5]

References:

  1. Niagara (1953, Henry Hathaway, Director) on the Internet Movie Database.
  2. Wonderfalls (2004, Television Series, Bryan Fuller, Creator) on the Internet Movie Database.
  3. P. A. S. Breslin and G. K. Beauchamp, "Salt enhances flavour by suppressing bitterness," Nature, vol. 387, no. 6633 (June 5, 1997), pp. 563 ff., doi:10.1038/42388.
  4. Russell Keast, Paul Breslin and Gary Beauchamp, "Suppression of bitterness using sodium salts," Chimia: International journal for chemistry, vol. 55, no. 5 (2001), pp. 441-447.
  5. Rebecca Phillips, "Green highway snow and ice control cuts the chemicals," Washington State University Press Release, November 19, 2014.

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