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.
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 (MgCl
2),
calcium chloride (CaCl
2), 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, (MgCl
2) and calcium chloride (CaCl
2) have more "
bang-for-the-buck" when used to melt ice.
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]
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:
- Niagara (1953, Henry Hathaway, Director) on the Internet Movie Database.
- Wonderfalls (2004, Television Series, Bryan Fuller, Creator) on the Internet Movie Database.
- 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.
- 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.
- Rebecca Phillips, "Green highway snow and ice control cuts the chemicals," Washington State University Press Release, November 19, 2014.
