Zinc occurs abundantly in rocks and ores. It is readily refined into a stable pure metal and it is used extensively for galvanizing and alloys and other industrial manufacture.
Many of the zinc salts are highly soluble in water. Hence, it is to be expected that zinc might occur in effluents from industrial waters. On the other hand, some zinc salts are insoluble in water and consequently it is to be expected that some zinc will precipitate and be removed readily in most natural waters. There is some evidence that zinc ions are absorbed strongly and permanently on silt with a resultant inactivation of the zinc.
In zinc mining areas, it has been found in natural waters in concentrations as high as 50 mg/L. In effluents from metal plating works and ammunitions plants it may occur in significant concentrations. However, in most surface and ground waters it is present only in trace amounts.
Zinc has no adverse physiological effects upon man except at very high concentrations. In fact, zinc is an essential and beneficial element in human nutrition. The normal human intake of zinc is estimated at 10 to 15 mg per day. Also, small amounts of zinc are needed for nutrition for most crops, but toxicity results when concentration exceeds a very low level. A deficiency of zinc causes poor growth.
It is towards fish and aquatic organisms that zinc exhibits its greatest toxicity. In soft water, concentrations of zinc ranging from 0.1 to 1.0 mg/L have been reported to be lethal.
The presence of copper appears to have a synergistic effect on the toxicity of zinc. Zinc precipitated from solution in alkaline water showed little toxic action; almost all of the toxicity being attributed to the zinc remaining in solution.
However, calcium is antagonistic towards such toxicity. The lethal limit for zinc in water containing 1 mg/L of calcium is only 0.3 mg/L, but in water with 50 mg/L of calcium, as much as 2 mg/L is not toxic. This antagonistic effect of hardness toward zinc toxicity was confirmed by several investigators.
The toxicity of zinc salts to stickle backs in soft water is reduced by the addition of calcium chloride. Even more so than the addition of calcium carbonate. Hence, the calcium ion rather than the carbonate ion appears to be the antagonistic factor.
The toxicity of zinc salts is increased at lower concentrations of dissolved oxygen in about the same proportion as for lead, copper, and phenols. In other words, the lethal concentration at 60% saturation of dissolved oxygen is only about 0.85 that at 100% saturation.
Zinc is thought to exert its toxic action by forming insoluble compounds with the mucus that covers the gills and respiratory organs of aquatic organisms. The damage may be to the gill epithelium or possibly it may act as an internal poison.
The sensitivity of fish to zinc varies with species, age and condition of the fish as well as with the physical and chemical characteristics of the water. Some acclimatization to the presence of zinc is possible. Survivors from batches of fish subjected to dissolved zinc have been less susceptible to additional toxic concentrations than fish not previously exposed.
It has been observed that the effects of zinc poisoning may not become apparent immediately so that fish removed from zinc contaminated to zinc free water may die 48 hours later.
The zinc cyanide complex, unlike nickel cyanide, disassociates in very dilute solutions. Zinc has been found to be even more toxic than comparable solutions of cyanide without zinc, apparently because of synergism.
Finally, zinc has been shown to have a toxic effect toward protozoa and bacteria, but not nearly as pronounced as copper.