Use of Bark to Remove Heavy Metal Ions from Waste Solutions

Toxic heavy metal ions including copper, lead, cadmium, chromium, and mercury present a potential danger in industrial waste waters. Few of the methods now used or proposed to remove heavy metals from waste water– precipitation with a variety of organic compounds, ion exchange (though synethetic ion exchange resins are relatively expensive), and bacteriologically produced sulfide–are adequate to satisfy strict water quality limits, usually much less than 1 ppm for each heavy metal. Wood bark, high in tannins, should be ideal adsorbents. Metals are bound to bark substrates by ion exchange with H, presumably from the phenolic groups in tannin compounds. Bark of Lawson cypress, Douglas-fir, coastal redwood, red oak, western hemlock, and sweet gum were tested, and several were compared with peanut skins, known to be an efficient scavenger. All barks except Douglas-fir were relatively efficient in removing both lead acetate and lead nitrate from solution, but none except redwood could remove significant amounts of Cu, DI , Ag, or Zn. Tannins in other barks may be so water soluble that they are leached into the waste water. Red oak and western hemlock bled color into solution, imposing a BOD load on the aqueous effluent and inhibiting their used in pollution abatement. After initial tests of several types of barks and heavy metal ions, the efficiency of redwood bark was tested using actual waste water. Runoff water from a copper mine having a cupric ion concentration of 6.0 ppm and a pH of 6.33 was passed through a 30 cm bed fo ground redwood bark; Cu in the effluent was easily reduced to less than 0.1 ppm throughout the run. When an acid sample from a lead battery plant with a pH 2.33 and a 4.5 ppm lead ion concentration was treated with redwood bark, residual Pb was reduced to less than 0.5 ppm; removal of Pb was improved by raising the pH. In this test the redwood bark not only removed ionic lead but also filtered out particulate matter present in the waste solution. All divalent heavy metal ions in aqueous solutions could be easily reduced below the maximum acceptable levels through use of redwood bark.

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