Copper is listed as a plant "micronutrient," which implies that it is not toxic, so long as the concentration is low. If the barrel was cleaned out, it should be safe to use for a compost bin. It is very unlikely that the plastic barrel would have absorbed copper sulfate to any significant degree. That said, there is no doubt that you can destroy your garden with copper sulfate, if you dump enough onto the soil.

Hi irontech. My short answer..I don't know. Copper sulfate is water soluable, so it would probably wash out of the barrel easily. A few things to consider are: Toxicological Effects:
Acute toxicity: Copper sulfate is caustic and acute toxicity is largely due to this property [8]. There have been reports of human suicide resulting from the ingestion of gram quantities of this material [21]. The lowest dose of copper sulfate that has been toxic when ingested by humans is 11 mg/kg [22]. Ingestion of copper sulfate is often not toxic because vomiting is automatically triggered by its irritating effect on the gastrointestinal tract. Symptoms are severe, however, if copper sulfate is retained in the stomach, as in the unconscious victim. Some of the signs of poisoning which occurred after 1 to 12 g of copper sulfate was swallowed include a metallic taste in the mouth, burning pain in the chest and abdomen, intense nausea, repeated vomiting, diarrhea, headache, sweating, shock, discontinued urination leading to yellowing of the skin. Injury to the brain, liver, kidneys, and stomach and intestinal linings may also occur in copper sulfate poisoning [23]. Copper sulfate can be corrosive to the skin and eyes [8]. It is readily absorbed through the skin and can produce a burning pain, as well as the other symptoms of poisoning resulting from ingestion. Skin contact may result in itching or eczema [8]. It is a skin sensitizer and can cause allergic reactions in some individuals [24]. Eye contact with this material can cause conjunctivitis, inflammation of the eyelid lining, cornea tissue deterioration, and clouding of the cornea [23]. Examination of copper sulfate poisoned animals showed signs of acute toxicity in the spleen, liver, and kidneys [8]. Injury may also occur to the brain, liver, kidneys, and gastrointestinal tract in response to overexposure to this material [22]. The oral LD50 of copper is 472 mg/kg in rats [8].
Chronic toxicity: Vineyard sprayers experienced liver disease after 3 to 15 years of exposure to copper sulfate solution in Bordeaux mixture [8]. Long term effects are more likely in individuals with Wilson's disease, a condition which causes excessive absorption and storage of copper [25]. Chronic exposure to low levels of copper can lead to anemia [8]. The growth of rats was retarded when given dietary doses of 25 mg/kg/day of copper sulfate. Dietary doses of 200 mg/kg/day caused starvation and death [8]. Sheep given oral doses of 20 mg/kg/day showed blood cell and kidney damage [8]. They also showed an absence of appetite, anemia, and degenerative changes [22].
Reproductive effects: Copper sulfate has been shown to cause reproductive effects in test animals. Testicular atrophy increased in birds as they were fed larger amounts of copper sulfate. Sperm production was also interrupted to varying degrees [8]. Reproduction and fertility was affected in pregnant rats given this material on day 3 of pregnancy [23].
Teratogenic effects: There is very limited evidence about the teratogenic effects of copper sulfate. Heart disease occurred in the surviving offspring of pregnant hamsters given intravenous copper salts on day 8 of gestation [8]. These data suggest that copper sulfate is unlikely to be teratogenic in humans at expected exposure levels.
Mutagenic effects: Copper sulfate may cause mutagenic effects at high doses. At 400 and 1000 ppm, copper sulfate caused mutations in two types of microorganisms [22]. Such effects are not expected in humans under normal conditions.
Carcinogenic effects: Copper sulfate at 10 mg/kg/day caused endocrine tumors in chickens given the material parenterally, that is, outside of the gastrointestinal tract through an intravenous or intramuscular injection [22]. However, the relevance of these results to mammals, including humans, is not known.
Organ toxicity: Long-term animal studies indicate that the testes and endocrine glands have been affected.
Fate in humans and animals: Absorption of copper sulfate into the blood occurs primarily under the acidic conditions of the stomach. The mucous membrane lining of the intestines acts as a barrier to absorption of ingested copper [8]. After ingestion, more than 99% of copper is excreted in the feces. However, residual copper is an essential trace element that is strongly bioaccumulated [8]. It is stored primarily in the liver, brain, heart, kidney, and muscles.
Ecological Effects:

Effects on birds: Copper sulfate is practically nontoxic to birds. It poses less of a threat to birds than to other animals. The lowest lethal dose (LDLo) is 1000 mg/kg in pigeons and 600 mg/kg in ducks [8]. The oral LD50 for Bordeaux mixture in young mallards is 2000 mg/kg [27].
Effects on aquatic organisms: Copper sulfate is highly toxic to fish [28]. Even at recommended rates of application, this material may be poisonous to trout and other fish, especially in soft or acid waters. Its toxicity to fish generally decreases as water hardness increases. Fish eggs are more resistant than young fish fry to the toxic effects of copper sulfate [26]. Copper sulfate is toxic to aquatic invertebrates, such as crab, shrimp, and oysters. The 96-hour LC50 of copper sulfate to pond snails is 0.39 mg/L at 20 C. Higher concentrations of the material caused some behavioral changes, such as secretion of mucous, and discharge of eggs and embryos [8].
Effects on other organisms: Bees are endangered by Bordeaux mixture [1]. Copper sulfate may be poisonous to sheep and chickens at normal application rates. Most animal life in soil, including large earthworms, have been eliminated by the extensive use of copper containing fungicides in orchards [28].
Environmental Fate:

Breakdown in soil and groundwater: Since copper is an element it will persist indefinitely. Copper is bound, or adsorbed, to organic materials, and to clay and mineral surfaces. The degree of adsorption to soils depends on the acidity or alkalinity of the soil [8]. Because copper sulfate is highly water soluble, it is considered one of the more mobile metals in soils. However, because of it binding capacity, its leaching potential is low in all but sandy soils [8]. When applied with irrigation water, copper sulfate does not accumulate in the surrounding soils. Some (60%) is deposited in the sediments at the bottom of the irrigation ditch, where it becomes adsorbed to clay, mineral, and organic particles. Copper compounds also settle out of solution [26].
Breakdown in water: As an element, copper can persist indefinitely. However, it will bind to water particulates and sediment.
Breakdown in vegetation: One of the limiting factors in the use of copper compounds is their serious potential for phytotoxicity [24]. Copper sulfate can kill plants by disrupting photosynthesis. Blue-green algae in some copper sulfate treated Minnesota lakes became increasingly resistant to the algacide after 26 years of use [28].

Sorry for the long answer,,,Good luck with your decision..

It is wise to be cautious with chemicals, even water can be toxic, if you drink too much, too quickly. Given that the plastic barrel has been thoroughly cleaned out, the remaining question is, has copper sulfate been absorbed into the plastic, and will it leach out, into the compost, over time? Plastics, in general, consist of long chain hydrocarbons. One would expect that hydrocarbons, either liquid or gas, might be absorbed into the plastic matrix. If the barrel had been used to store kerosene, olive oil, or methane, then it would be reasonable to expect that some of the contents had entered the plastic, and would leach out over time. It is unlikely that an inorganic salt, like copper sulfate, sodium chloride, or potassium cyanide, would be absorbed into a plastic container. Personally, I would use the barrel for making compost, but we have an open pile, so I have no need.

bill, bless his heart, kinda overkilled it with data there. :-]

I would agree with ericwi, the stuff would not absorb into the plastic in significant amounts, and is water soluble so if the barrel was rinsed out it will be fine. You didn't say whether it was previously washed, but CuSO4 looks like blue salt crystals. If you see that or rinse it out and get blue water, don't pour that on plants. Put it somewhere you want to kill weeds like sidewalk cracks.