Beneficial Effects of Aluminum

Although the essentiality of aluminum as a nutrient is questionable, aluminum compounds have been used for many years in animal agriculture, environmental management, and the food and pharmaceutical industries for beneficial purposes. In animals and humans, the beneficial effects usually occur at levels of aluminum intake far above that found in typical diets and, as such, in pharmacological treatments that may carry some risk of aluminum toxicity.

Beneficial Effects of Aluminum in Animal Agriculture

Aluminum is generally not added to animal diets because of the lack of any known nutritional function, and no evidence suggests beneficial effects occur in livestock grazing high-aluminum pastures. Rather, aluminum toxicity is of concern as some forages contain over 2000 mg Al kg^-1 (334). For a variety of useful reasons, however, aluminum compounds have been added to animal diets.

One of the oldest uses of aluminum compounds in agriculture is the use of bentonite clay (Al silicates of sodium, calcium, or other cations) as a binder for pelleted feeds. Studies in the 1950s with poultry indicated no detrimental effects of ingesting bentonite, and some indicated a beneficial effect on growth rate. Benefits were attributed to an increase in feed intake and a delay in the passage of feed through the digestive tract resulting in better absorption of nutrients (335). More recently, bentonite and other aluminosilicates have been investigated for their ability to ameliorate the toxic effects of aflotoxin-contaminated feeds on growth and feed intake in poultry and swine (336,337). Feeding hydrated sodium calcium aluminosilicates has also been shown to reduce the passage of aflatoxins into milk (338). The mechanism of action appears to be adsorption of aflatoxins by the aluminosilicates, reducing aflatoxin bioavailability.

The addition of aluminosilicates to poultry diets has also been reported to enhance eggshell quality (339). Feeding sodium zeolite A, a synthetic aluminosilicate with a 1:1 ratio of aluminum to silicon, increased the levels of silicon and aluminum in the blood. The authors suggested that the increase in blood silicon stimulated calcium use for eggshell formation. Wisser et al. (340), however, were able to show small increases in eggshell quality by adding aluminum sulfate to poultry diets, suggesting that aluminum had an effect independent of silicon. With aluminum sulfate, however, aluminum accumulated in the bones of the hens and reduced fertility. Similar, but less severe toxic effects were reported with sodium zeolite A, suggesting that zeolites may be a safer way to stimulate eggshell formation (341).

Sodium zeolite A has also been shown to prevent a condition referred to as milk fever (parturient hypocalcemia) in dairy cows, a relatively common problem in the dairy industry (342). Around the time of calving, the metabolic demand for calcium to support gestational growth and milk production is large. This demand for calcium can result in hypocalcemia leading to muscle tremors, weakness, and eventually death if not treated. Sodium zeolite A added to the ration for 3 weeks prior to calving was found to stimulate calcium mobilization from bone and enhance the efficiency of calcium absorption, preventing hypocalcemia (342). The stimulus for these changes in calcium metabolism appeared to come from an aluminum-induced reduction in phosphate availability, since treated cows had significantly lower plasma inorganic phosphate levels.

Similar to the above concept of using aluminum to inhibit phosphate absorption, aluminum has been shown to inhibit fluoride absorption and protect against fluoride toxicity in poultry (343). Aluminum fluoride complexes may be formed in the body, however, and may have detrimental effects of their own (344). Aluminum has also been studied for its beneficial effects on reducing lead toxicity (345).

Some of the beneficial roles of aluminum compounds in animal agriculture are unrelated to aluminum ingestion. Aluminum sulfate has been used to acidify poultry litter to reduce the growth and transmission of bacterial infections caused by Campylobacter. Campylobacter is a common cause of diarrhea in humans, and undercooked poultry is a potential source. In a recent study, litter contaminated with this bacterium was treated with aluminum sulfate, then, newly hatched chicks were raised on the treated litter (346). No transmission of Campylobacter to the chicks was observed. Unfortunately, the treatment was not effective against Salmonella. Aluminum compounds have also been used to treat animal manure prior to land applications to reduce environmental impacts.

Beneficial Uses of Aluminum in Environmental Management and Water Treatment

The use of animal manures as fertilizers can increase water pollution problems due to runoff of soluble phosphorus. Several aluminum-containing compounds have been shown to reduce phosphate runoff if applied to manure. Applications of aluminum sulfate or aluminum chloride to swine manure reduced soluble phosphate in runoff by 84%, presumably by forming insoluble phosphate complexes (347). In a large scale, on-farm trial, aluminum sulfate was applied over a 16-month period to litter in 97 poultry houses on the Delmarva Peninsula. Compared to litter from untreated houses, treated litter had decreased soluble phosphates, a lower pH, and higher total nitrogen and sulfur concentrations, thereby increasing its value as a fertilizer (348). Zeolite and aluminum sulfate were evaluated in amending slurries of dairy manure (349). Aluminum sulfate eliminated soluble phosphorus, and zeolite reduced it by over half. Both aluminum compounds reduced ammonia emissions by 50%, presumably by reducing the pH or by adsorbing ammonium cations. Peak et al. (350) used x-ray absorption near edge spectroscopy to determine the chemical species of aluminum and phosphorus in treated manures. No evidence of aluminum phosphate precipitation was found. Therefore, the mechanism of action is not clear and brings up the possibility that soluble forms of aluminum may be present in the treated manures and, hence, in the runoff, especially if excess aluminum is used in the treatment process.

Aluminum sulfate also has been used to treat algal-rich, eutrophied lakes. Welch and Cooke (351) reported the effectiveness and longevity of treatments in 21 lakes across the United States. They concluded that aluminum sulfate effectively reduced total soluble phosphate levels (and the algae that depend on this nutrient) for 8 years on average, especially in lakes without large external inputs of phosphorus. Aluminum is thought to form insoluble aggregates of aluminum phosphate, hydroxide, and organic material that settle to the bottom of the lake and remain in the sediment unless solubilized by acidic conditions. Acid conditions release soluble forms of aluminum that can be toxic to fish, prompting guidelines that lake pH should remain between 5.5 and 9.0.

Very little evidence suggests that aluminum is beneficial to aquatic species under normal circumstances. Short-term protective effects of aluminum against acid (H⁺) toxicity have been shown in some studies (352). Uptake of protons from acidic water can fatally disrupt electrolyte regulation in fish. However, under acidic conditions, monomeric aluminum (Al
3) may bind to gill surfaces blocking the binding and systemic uptake of H⁺, thereby improving survival. This protective effect may only last a few hours and has been reported only under laboratory conditions. Aluminum in acidic water (pH 5.2 to 5.9) was also shown to eliminate ectoparasites on Atlantic salmon better than acidic water alone (353).

Municipal water treatment facilities often use aluminum sulfate as a water-clarifying agent in a process similar to that described above for treating eutrophied lakes. The basic process is ancient, originating in China thousands of years ago. When aluminum sulfate is added to turbid water at pH 6.5 to 8, aluminum hydroxide forms as a gel-like precipitate (floc). Suspended particles and oils are trapped in the floc, which is then removed by various methods. Some aluminum, however, can remain in solution. Concentrations of aluminum in treated drinking water have ranged from undetectable to 2.7 mg L^-1, with a median of 0.1 mg L^-1 (354). The Environmental Protection Agency has suggested a maximum contamination level for aluminum in drinking water at a concentration range of 0.05 to 0.2 mg L^-1. Recently, other types of aluminum-based clarifying agents such as polyaluminum chloride have been used that may result in less residual aluminum and different chemical species of residual aluminum in treated water compared to current methods (355,356). Clarification of water by aluminum compounds has been investigated for its potential to reduce drinking water fluoride concentrations in regions where fluoride toxicity is a concern (357).