D
Several of the so-called trace metals fill essential roles in life processes— e.g., manganese, iron, cobalt, copper,
etection of mercury in water and in fish life above the standards established by the U.S. Public Health Service and growing concern with lead as an environmental contaminant stemming from its wide usage as a gasoline additive have sparked general interest in heavy metals as potential hazards in environmental control. The metals of most immediate concern are: chromium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, mercury, and lead. These metals are widely distributed in materials which make up the earth’s surface. Igneous rocks, for example, typically average about 5% iron and contain other heavy metals at various levels ranging down to about 20 ppm lead, which is a relatively scarce element. These rocks are constantly weathered and leached by rainwater, and yet the natural runoff in rivers, even in extreme cases such as the Colorado River, is remarkably free of dissolved heavy metals. courses
zinc, and molybdenum, while others such as mercury and lead are regarded with more suspicion as potential cumulative poisons. A remarkable compatibility exists, however, between the chemistry of heavy metals as encountered in nature and living organisms. Heavy metals occur largely in natural mineral form as sulfides, oxides, carbonates, and silicates. These natural compounds are usually insoluble in water and only very slowly broken down by weathering and exposure to rainfall and groundwaters. For example, rainwater containing dissolved carbon dioxide attacks basic rocks such as peridotite which may contain 50% magnesium oxide and selectively dissolves magnesium in association with the bicarbonate ion, while iron, which may be similarly dissolved, oxidizes to the ferric form and precipitates as highly insoluble ferric hydrate even
Removing
metals heavy from waste water
518
Environmental Science & Technology
John G. Dean and Frank L. Bosqui
feature
Dean Associates
North Scituate, Rl 02857
Kenneth H. Lanouette Industrial Pollution Control, Inc. Westport, CT 06880
Pending water pollution legislation requires heavy metal removal not only before industrial wastes are discharged into
navigable waters, but also prior to
when pH is as low as 2. Other heavy metals tend similarly to follow the behavior of iron and precipitate in the oxide residue while magnesia is carried off in groundwater as bicarbonate hardness. The order of precipitation from dilute solutions as the pH is raised is as
follows: Ion
pH
Ion
pH
Fe3+
2.0 4.1 5.3 5.3 5.5 6.0
Na2+ Cd2+ Zn2+
6.7 6.7 7.0 7.3 8.5 6.9
Al3+ Cr3+ Cu2+ Fe2+ Pb2+
Hg2Mn2+ Co2+
Very small heavy metal ion concentrations can be expected in neutral solutions. Ferric ion concentration in neutral water solution may even be far below the parts per billion (ppb) range subject to practical measurement. Iron is an extreme case but even lead shows extreme insolubility; for example, lead ion concentration at equilibrium with water containing 5 ppm or more of carbonate ion is less than 1 jug/1. or 1 ppb. The heavy metals are, for the most
part, responsive to practical treatment methods which have already been developed and utilized for water purification and metal recovery operations. Treatment methods which should be considered include chemical precipitation, cementation, electrodeposition, solvent extraction, reverse osmosis, and ion exchange.
Chemical precipitation The most generally applied treatment method, particularly where complex chemical compounds are not involved and economic recovery is not a con-
ocean
or
sideration, is the typical lime treatment plant (Figure 1), because of its relative simplicity and low cost of precipitant. Removing such metals as copper, zinc, iron, manganese, nickel, and cobalt requires almost complete precipitation as the hydroxide with no special modifications. For cadmium, lead, and mercury, precipitation may be incomplete, however, and a modified flowsheet employing soda ash (for lead) or sodium sulfide (for cadmium and mercury) may be required. Where chromium is present, reducing the solution with sulfur dioxide, ferrous sulfate, or metallic iron before lime treatment is necessary. Chlorination may be needed to break down complex organic metallic compounds before chemical precipitation. Where strong acidic wastes exist, part of the neutralization with limestone may be somewhat less expensive than lime. However, limestone must be evaluated carefully for each acid waste since it may not be effective as theoretically indicated owing to particle coating, need for fine grinding, and pH limitation of calcium carbonate. For example, nickel sulfate-sulfuric acid solutions treated first with crushed limestone, then with hydrated lime, show: Soln A
NiS04, meq/1.
4
H2S04, meq/1. Nickel, ppm
4 117
pH
1.8
Soln B
100 100
2,935 0.
Agitation in a rotating reactor for 30 min with —10 + 40-mesh crushed limestone at a rate of 100 g/1., filtered:
pH Nickel
4.9
4.9
unchanged unchanged
land disposal
Agitation of filtrate with two times equivalent of the nickel with calcium hydroxide for 30 min, filtered: pH
Ni,ppm
10.0
