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← A circular economy for salt that keeps rivers clean

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Showing Revision 8 created 10/16/2019 by Brian Greene.

  1. Growing up in northern Wisconsin,
  2. I've naturally developed a connection
    to the Mississippi River.
  3. When I was little,
  4. my sister and I would have contests
    to see who could spell
  5. "M-i-s-s-i-s-s-i-p-p-i" the fastest.
  6. When I was in elementary school,
  7. I got to learn about the early explorers
    and their expeditions,
  8. Marquette and Joliet, and how they used
    the Great Lakes and the Mississippi River
  9. and its tributaries
    to discover the Midwest,
  10. and to map a trade route
    to the Gulf of Mexico.
  11. In graduate school,
  12. I was fortunate to have
    the Mississippi River
  13. outside my research laboratory window
  14. at the University of Minnesota.
  15. During that five-year period,
    I got to know the Mississippi River.
  16. I got to know its temperamental nature
  17. and where it would flood
    its banks at one moment,
  18. and then shortly thereafter,
  19. you would see its dry shorelines.
  20. Today, as a physical organic chemist,

  21. I'm committed to use my training
  22. to help protect rivers,
    like the Mississippi,
  23. from excessive salt
    that can come from human activity.
  24. Because, you know,
  25. salt is something that can contaminate
    freshwater rivers.
  26. And freshwater rivers,
    they have only salt levels of .05 percent.
  27. And at this level, it's safe to drink.
  28. But the majority of the water
    on our planet is housed in our oceans,
  29. and ocean water has a salinity level
    of more than three percent.
  30. And if you drank that,
    you'd be sick very quick.
  31. So, if we are to compare
    the relative volume of ocean water
  32. to that of the river water
    that's on our planet,
  33. and let's say we are able
    to put the ocean water
  34. into an Olympic-size swimming pool,
  35. then our planet's river water
    would fit in a one-gallon jug.
  36. So you can see it's a precious resource.
  37. But do we treat it
    like a precious resource?

  38. Or rather, do we treat it
    like that old rug
  39. that you put in your front doorway
    and wipe your feet off on it?
  40. Treating rivers like that old rug
    has severe consequences.
  41. Let's take a look.
  42. Let's see what just one teaspoon
    of salt can do.
  43. If we add one teaspoon of salt
  44. to this Olympic-size
    swimming pool of ocean water,
  45. the ocean water stays ocean water.
  46. But if we add that same
    one teaspoon of salt
  47. to this one-gallon jug
    of fresh river water,
  48. suddenly, it becomes too salty to drink.
  49. So the point here is,
  50. because rivers, the volume is so small
    compared to the oceans,
  51. it is especially vulnerable
    to human activity,
  52. and we need to take care to protect them.
  53. So recently, I surveyed the literature

  54. to look at the river health
    around the world.
  55. And I fully expected to see
    ailing river health
  56. in regions of water scarcity
    and heavy industrial development.
  57. And I saw that
    in northern China and in India.
  58. But I was surprised
    when I read a 2018 article
  59. where there's 232 river-sampling sites
  60. sampled across the United States.
  61. And of those sites,
  62. 37 percent had increasing salinity levels.
  63. What was more surprising
  64. is that the ones
    with the highest increases
  65. were found on the east part
    of the United States,
  66. and not the arid southwest.
  67. The authors of this paper postulate
  68. that this could be due
    to using salt to deice roads.
  69. Potentially, another source of this salt
  70. could come from salty
    industrial wastewaters.
  71. So as you see, human activities
    can convert our freshwater rivers

  72. into water that's more like our oceans.
  73. So we need to act and do something
    before it's too late.
  74. And I have a proposal.
  75. We can take a three-step
    river-defense mechanism,

  76. and if industrial-water users
    practice this defense mechanism,
  77. we can put our rivers
    in a much safer position.
  78. This involves, number one,
  79. extracting less water from our rivers
  80. by implementing water recycle
    and reuse operations.
  81. Number two,
  82. we need to take the salt
    out of these salty industrial wastewaters
  83. and recover it and reuse it
    for other purposes.
  84. And number three,
    we need to convert salt consumers,
  85. who currently source our salt from mines
  86. into salt consumers that source our salt
    from recycled salt sources.
  87. This three-part defense mechanism
    is already in play.
  88. This is what northern China
    and India are implementing
  89. to help to rehabilitate the rivers.
  90. But the proposal here
  91. is to use this defense mechanism
    to protect our rivers,
  92. so we don't need to rehabilitate them.
  93. And the good news is,
    we have technology that can do this.
  94. It's with membranes.
  95. Membranes that can separate
    salt and water.

  96. Membranes have been around
    for a number of years,
  97. and they're based on polymeric materials
    that separate based on size,
  98. or they can separate based on charge.
  99. The membranes that are used
    to separate salt and water
  100. typically separate based on charge.
  101. And these membranes
    are negatively charged,
  102. and help to repel the negatively
    charged chloride ions
  103. that are in that dissolved salt.
  104. So, as I said, these membranes
    have been around for a number of years,
  105. and currently, they are purifying
    25 million gallons of water every minute.
  106. Even more than that, actually.
  107. But they can do more.
  108. These membranes are based
    under the principle of reverse osmosis.

  109. Now osmosis is this natural process
    that happens in our bodies --
  110. you know, how our cells work.
  111. And osmosis is where you have two chambers
  112. that separate two levels
    of salt concentration.
  113. One with low salt concentration
  114. and one with high salt concentration.
  115. And separating the two chambers
    is the semipermeable membrane.
  116. And under the natural osmosis process,
  117. what happens is the water naturally
    transports across that membrane
  118. from the area of low salt concentration
  119. to the area of high salt concentration,
  120. until an equilibrium is met.
  121. Now reverse osmosis,
    it's the reverse of this natural process.

  122. And in order to achieve this reversal,
  123. what we do is we apply a pressure
    to the high-concentration side
  124. and in doing so, we drive the water
    the opposite direction.
  125. And so the high-concentration side
    becomes more salty,
  126. more concentrated,
  127. and the low-concentration side
    becomes your purified water.
  128. So using reverse osmosis,
    we can take an industrial wastewater
  129. and convert up to 95 percent of it
    into pure water,
  130. leaving only five percent
    as this concentrated salty mixture.
  131. Now, this five percent
    concentrated salty mixture
  132. is not waste.
  133. So scientists have also
    developed membranes
  134. that are modified to allow
    some salts to pass through
  135. and not others.
  136. Using these membranes,
  137. which are commonly referred to
    as nanofiltration membranes,
  138. now this five percent
    concentrated salty solution
  139. can be converted
    into a purified salt solution.
  140. So, in total, using reverse osmosis
    and nanofiltration membranes,
  141. we can convert industrial wastewater
  142. into a resource of both water and salt.
  143. And in doing so,
  144. achieve pillars one and two
    of this river-defense mechanism.
  145. Now, I've introduced this
    to a number of industrial-water users,

  146. and the common response is,
  147. "Yeah, but who is going to use my salt?"
  148. So that's why pillar number three
    is so important.
  149. We need to transform folks
    that are using mine salt
  150. into consumers of recycled salt.
  151. So who are these salt consumers?
  152. Well, in 2018 in the United States,
  153. I learned that 43 percent of the salt
    consumed in the US
  154. was used for road salt deicing purposes.
  155. Thirty-nine percent
    was used by the chemical industry.
  156. So let's take a look
    at these two applications.

  157. So, I was shocked.
  158. In the 2018-2019 winter season,
  159. one million tons of salt
  160. was applied to the roads
    in the state of Pennsylvania.
  161. One million tons of salt is enough
  162. to fill two-thirds
    of an Empire State Building.
  163. That's one million tons of salt
    mined from the earth,
  164. applied to our roads,
  165. and then it washes off
    into the environment and into our rivers.
  166. So the proposal here
    is that we could at least
  167. source that salt from a salty
    industrial wastewater,
  168. and prevent that
    from going into our rivers,
  169. and rather use that to apply to our roads.
  170. So at least when the melt happens
    in the springtime
  171. and you have this high-salinity runoff,
  172. the rivers are at least
    in a better position
  173. to defend themselves against that.
  174. Now, as a chemist,

  175. the opportunity though
    that I'm more psyched about
  176. is the concept of introducing
    circular salt into the chemical industry.
  177. And the chlor-alkali industry is perfect.
  178. Chlor-alkali industry
    is the source of epoxies,
  179. it's the source of urethanes and solvents
  180. and a lot of useful products
    that we use in our everyday lives.
  181. And it uses sodium chloride salt
    as its key feed stack.
  182. So the idea here is,
  183. well, first of all,
    let's look at linear economy.
  184. So in a linear economy,
    they're sourcing that salt from a mine,

  185. it goes through this chlor-alkali process,
  186. made into a basic chemical,
  187. which then can get converted
    into another new product,
  188. or a more functional product.
  189. But during that conversion process,
  190. oftentimes salt is regenerated
    as the by-product,
  191. and it ends up
    in the industrial wastewater.
  192. So, the idea is that we can
    introduce circularity,
  193. and we can recycle the water and salt
    from those industrial wastewater streams,
  194. from the factories,
  195. and we can send it to the front end
    of the chlor-alkali process.
  196. Circular salt.
  197. So how impactful is this?

  198. Well, let's just take one example.
  199. Fifty percent of the world's
    production of propylene oxide
  200. is made through the chlor-alkali process.
  201. And that's a total of about five million
    tons of propylene oxide
  202. on an annual basis, made globally.
  203. So that's five million tons of salt
    mined from the earth
  204. converted through the chlor-alkali process
    into propylene oxide,
  205. and then during that process,
  206. five million tons of salt
    that ends up in wastewater streams.
  207. So five million tons
  208. is enough salt to fill
    three Empire State Buildings.
  209. And that's on an annual basis.
  210. So you can see how circular salt
    can provide a barrier
  211. to our rivers from this excessive
    salty discharge.
  212. So you might wonder,

  213. "Well, gosh, these membranes
    have been around for a number of years,
  214. so why aren't people implementing
    wastewater reuse?
  215. Well, the bottom line is,
  216. it costs money to implement
    wastewater reuse.
  217. And second,
  218. water in these regions is undervalued.
  219. Until it's too late.
  220. You know, if we don't plan
    for freshwater sustainability,
  221. there are some severe consequences.
  222. You can just ask one of the world's
    largest chemical manufacturers
  223. who last year took
    a 280-million dollar hit
  224. due to low river levels
    of the Rhine River in Germany.
  225. You can ask the residents
    of Cape Town, South Africa,
  226. who experienced a year-over-year drought
    drying up their water reserves,
  227. and then being asked
    not to flush their toilets.
  228. So you can see,

  229. we have solutions here, with membranes,
  230. where we can provide pure water,
  231. we can provide pure salt,
  232. using these membranes, both of these,
  233. to help to protect our rivers
    for future generations.
  234. Thank you.

  235. (Applause)