ORIGINAL RESEARCH: Removal of heavy metals from polluted waters using lignocellulosic agricultural waste products

Many agricultural wastes, including barks, manures, and composts, contain high levels of lignocellulosic materials. We hypothesized that the lignin fraction, which contains numerous reactive groups, would be highly effective in binding and removing heavy metal ions from contaminated water, and, further, that the absorptive capabilities of the materials would be strongly affected by the pH of the solution. A series of materials were tested, and, at pH levels above about 5.5, they were highly effective in removing heavy metal ions, generally as large or larger than nickel, but ineffective in removal of lighter ions such as sodium or magnesium. Various barks were generally the most effective and were capable of removing more than 90% of iron, copper, or lead from solutions in simple shake flask experiments. Materials that retain cellular structures and that have high lignin contents were highly effective; barks possess these properties. At alkaline pH levels, many heavy metal ions precipitate, but three separate lines of evidence indicate that, in the work reported here, ions were removed from aqueous solutions by absorption to barks rather than by precipitation. At acidic pH levels, they also were partially effective in removal of the oxyanion chromate. However, a more efficient method of removal of chromate involved reduction to chromite in acidic solutions, adjusting pH levels in transit to columns packed with bark, and removal of chromite with hardwood bark. The bark was capable of binding the chromite to a level more than 3% by weight, and the resulting bark-chromite mixture passed toxicity characteristic leaching procedure (TCLP) tests. This system is expected to be deployed to remediate a site with chromate-polluted groundwater.