Removal of Heavy Metal Ions from Waste Solutions
by Contact with Agricultural Byproducts
JOHN M. RANDALL, Research Chemical Engineer
EARL HAUTALA, Research Chemist
Agricultural Research Service
Western Regional Research Laboratory
U.S. Department of Agriculture
Berkeley, California 94710
INTRODUCTION
Federal and local agencies have imposed increasingly stringent limits on the quantities
of heavy metal ions in waste streams. Among the toxic heavy metal ions which present a
potential health hazard to aquatic, animal, and human life are mercury, lead, cadmium, and
copper. It has been reported that lead and cadmium are perhaps the two most dangerous
heavy metal ions in our environment, primarily because they are so widespread (1).
However, there are fairly stringent limits on the concentration of other heavy metal ions,
including iron, chromium, etc., allowed in waste streams.
Precipitation of Cu++, Pb++ and Fe+++ as hydroxide or other salts is often used
industrially to reduce the concentrations of these ions in solution (2). With Fe+++, however,
the resulting floe of ferric hydroxide is difficult to settle or filter. Precipitation of heavy
metal ions has also been attempted with a number of organic compounds (3, 4) and with
bacteriologically produced sulfides (5). Few of the methods currently being used to remove
heavy metal ions from solution can consistently meet current strict water quality standards.
Interest has risen recently in removing heavy metal ions from solution by binding with
such agricultural materials such as waste wool (6), nut wastes (7), tree barks (8), and
modified cotton (9). Other natural byproducts such as chitosan (produced by deacetylation
of chitin) and milorganite also show promise in binding heavy metal ions (10). Many
agricultural byproducts are of little or no economic value, and some, such as nut wastes and
barks, often present a disposal problem. This paper will discuss some of the experimental
work carried out on the binding of heavy metal ions to agricultural substrates and describe a
current demonstration project using calcium carbonate, a Uniflow filter, and a redwood
bark column to remove lead and iron from the waste effluent of a lead battery plant.
SORPTION OF HEAVY METAL IONS
In Table I are some agricultural materials which have been studied, separated into their
relative affinities for heavy metal ions. The nut wastes and the effective barks tend to be high
in tannins, and wool has multifunctional binding groups. Tannin compounds contain many
linked phenyl rings with adjacent hydroxy groups, very similar to catechol which is well
known for forming a chelate with Cu++. The substrates which display little affinity for
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