Arsenic in the Soil Environment: A Review

Although it is now commonly accepted that toxicity and bioavailability varies with As species, extensive research has been carried out on biological and environmental samples to assess toxicity and risk associated with As exposure based on total concentrations that may be in error. The health investigation guideline for the Australian environmental protection measure is 100 mg/kg (As(tot)), which would cause potential risk to human health if all the As present in a sample were bioavailable (ANZECC 1992). Similarly, the MPC for As in food is 1 mg/ kg (fresh weight), but this concentration may include contributions from As(III), AsV, and all organic species. Thus, a food substance, such as seafood, could have a high total concentration exceeding the guidelines, but most of the As would be in forms that are nontoxic to humans; i.e., the bioavailability is low, and the food would therefore be perfectly safe to eat. On the other hand, a food that has high bioavailability of As consequently is more toxic. Overall, it appears that contamination of water by As is probably more harmful to humans than As in food grains or vegetables, because As bioavailability in water is generally higher than its bioavailability in food. Nevertheless, As in food crops could make significant contribution toward total daily intake. Therefore, failure to consider the contribution of As species on their bioavailability could introduce a substantial bias into the estimation of risks associated with exposure as well as evaluation of As toxicity. In conclusion, As must be regarded as an important environmental toxicant because of its acute and chronic toxic properties and extensive presence in the environment. Much remains to be learned about its toxicology and biochemistry for better understanding of this important contaminant.

Arsenic speciation and toxicity in biological systems.

(1986). Clinical and Environmental Aspects of Arsenic Toxicity. CRC Critical Reviews in Clinical Laboratory Sciences: Vol. 23, No. 4, pp. 315-347.

Clinical and environmental aspects of arsenic toxicity.

An overview is presented of the current state of knowledge of the salient aspects of the sources, transport, and alterations of arsenic, beryllium, cadmium, chromium, and nickel. This information is considered vital for a better assessment of the scope of potential human hazard to these ubiquitous toxicants and their compounds. Stress is focused on both natural and industrial activities, particularly on the latter's projected trends. Increasing use patterns per se of most of these metals, as well as aspects of waste disposal and the anticipated increased combustion of fossil fuels for power generation and space heating (particularly in the United States), are major causes of potential health concern. Additionally, attention is drawn to the need for increased research to fill the gaps in our knowledge in these vital areas, all in the hope of permitting a more facile identification and quantification of the potential hazard to exposure to these agents.

Sources, transport and alterations of metal compounds: an overview. I. Arsenic, beryllium, cadmium, chromium, and nickel.

Humans have been in contact with metals almost since the beginning of our existence. In fact, one cannot even think on human evolution without considering the great role played by metals in mankind's development. Metals are common moieties of molecules involved in a wide variety of biological processes, and hence are found in virtually all living organisms. Some metals are essential for human nutrition; others are found as contaminants in foodstuffs. One feature of the normal human diet which is frequently found is the simultaneous presence of both essential and toxic metals. Other factors important in the risk-evaluation analysis of metals are their pharmacokinetics, interactions among them and with other major components of the diet, and, especially, the great differences in the dietary habits of different populations and in the regional distribution of metals. In attempting to understand the role which dietary metals could play in human carcinogenesis, we found that the many factors involved and the lack of specific information made it difficult to reach firm conclusions on the hazards of dietary metals. We hope that this paper will raise the interest of genetic toxicologists in the subject and will consequently facilitate a risk analysis of the carcinogenic potential of dietary metals.

Are metals dietary carcinogens

Inorganic arsenicals have been used in agriculture as pesticides or defoliants for many years and, in localized areas, oxides of arsenic have contaminated soils as a result of fallout from ore-smelting operations and coal-fired power plants. Use of inorganic arsenicals is no longer permitted in most agricultural operations, and recent air pollution controls have markedly reduced contamination from smelters. Thus, this paper will concentrate on the effect of past applications on arsenic accumulation in soil, phytotoxicity to and uptake by plants as influenced by soil properties, and alleviation of the deleterious effects of arsenic. Once incorporated into the soil, inorganic arsenical pesticides and arsenic oxides revert to arsenates, except where the soil is under reducing conditions. The arsenate ion has properties similar to that of orthophosphate, and is readily sorbed by iron and aluminum components. This reaction greatly restricts the downward movement (leaching) of arsenic in soils and the availability of arsenic to plants. Several methods of estimating plant available arsenic in soils have been developed. They involve extraction of the soil with reagents used to estimate phosphorus availability. This extractable arsenic is reasonably well correlated with reduced plant growth by, and plant uptake of arsenic. For most plants, levels of arsenic in the edible portion of the plant are well below the critical concentration for animal or human consumption, even when severe phytotoxicity occurs. Alleviation of arsenic phytotoxicity has been attempted by increasing the soil pH, by use of iron or aluminum sulfate, by desorbing arsenate with phosphate and subsequent leaching, and by cultural practices such as deep plowing. Only limited benefits have accrued from these procedures the cost of which is often prohibitively high. Since attempts to reduce arsenic toxicity have not been very successful, its excessive accumulation in soils should be avoided.

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Malcolm E. Sumner

Papers

Occurrence and distribution of arsenic in soils and plants.