58    TREATMENT OF METAL-CONTAINING WASTEWATERS BY
CARBON ADSORPTION OF METAL-CHELATE
COMPLEXES
Margaret A. Shay, Project Engineer
Heritage Remediation/Engineering, Inc.
Indianapolis, Indiana 46241
James E. Etzel, Professor Emeritus
School of Civil Engineering
Department of Environmental Engineering
Purdue University
West Lafayette, Indiana 47907
INTRODUCTION
Heavy metals are present in many industrial wastewaters. In 1983, an estimated 7.9 billion gallons
of heavy metal bearing wastewaters were generated in the United States.1 Heavy metals which are
present in industrial wastewater above defined legal limits must be removed before discharge to a
receiving stream or a publicly owned treatment works.
The most common technique for removal of heavy metals from industrial wastewater is by precipitation as the hydroxide or sulfide salt. This method is used by approximately 75% of the electroplating facilities treating heavy metal bearing wastewaters.1 Most facilities use hydroxide as the precipitant. Disadvantages include solubility constraints of the heavy metal hydroxides, difficulty with
stabilizing heavy metal hydroxide sludge to make it non-leachable, ineffective removal of mixed heavy
metals due to differing minimum hydroxide solubilities, and interferences associated with complexing
agents such as EDTA.
To eliminate difficulties associated with complexing agents, this project investigated the feasibility
of a process which utilized chelating agents in the removal of heavy metals. Heavy metal ions were
removed from wastewater by adsorption of heavy metal-chelating agent complexes onto activated
carbon. In this process, chelating agents which might be present in the wastewater would enhance
removal of heavy metals, rather than interfere.
EXPERIMENTAL METHODS
Column tests were run to determine whether chelated heavy metals could be removed from solution
by adsorption onto activated carbon. Fifty-milliliter burets were filled to the 25-mL mark with
activated carbon, for a total carbon height of approximately 12 inches.
The test heavy metal solution was run through the column at a selected contact time. The test
solutions were prepared within 24 hours of running the experiment from a stock solution of heavy
metal sulfate (Cu, Ni, or Zn) that was diluted with deionized (DI) water to the desired metal concentration. The concentration was checked by atomic adsorption spectroscopy (AAS). Chelating agent
was added in a given molar ratio to the test heavy metal solution and the mixture stirred until the
chelating agent dissolved. Time was allowed for the reactions between the heavy metal and the
chelating agent to occur before the experiments were run. The solution was then passed through the
carbon column at a pre-selected flow rate, regulated by adjusting the buret stopcock. All the studies
reported were conducted at room temperature (20° to 23°C). No attempt was made to investigate the
effect of temperature on the removal of heavy metals.
The effluent from the column was collected in fractions and analyzed for heavy metals by AAS,
employing Standard Methods Part 300.2 The heavy metals analyses were done immediately following
the test to minimize errors due to evaporation or wall adsorption. If it was necessary to store the
samples, a small amount of concentrated HNOj was added.2
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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