FLUORIDES AND SULFUR DIOXIDES AS CAUSES OF PLANT DAMAGE

H. Bohne *
Bonn-Bad Godesberg, Germany

*From the Agricultural Chemical Laboratory, Bad Godesberg, Germany.

SUMMARY: Fluoride (F-) and sulfur trioxide (S03) levels were determined in three polluted industrial areas where the two pollutants had damaged vegetation. The appearance of the leaves was typical of F- injury whereas no lesions indicated toxicity by sulfur oxides. Chemical analyses of the plants supported these findings. Fluoride levels in gladiolus ranged from 3.4 to 8.7 mg% and those of S03 from 0. 77 to 1. 15%. The author concludes that, by far, the greatest damage from gaseous combustion to horticulture, farming and forestry is caused by F- containing compounds, primarily hydrogen fluoride.

In the extensive literature on damage to vegetation by air pollution, sulfur dioxide is considered to be the most widely distributed and destructive agent (1-3). This concept originated in the 19th century when damage appeared on forest plants near industries where large amounts of coal or coke were used. At that time it was already known that, during combustion, sulfur in coal is transformed into S02 which passes through the smokestack into the atmosphere and reaches the forests. In almost all cases, the portion of the trees facing the factories was more heavily damaged than that on the opposite sides. Therefore, it was thought that injury to trees had been brought about by emissions of S02.

This conclusion was further supported by the fact that the damaged portions of plants near factories contain considerably more sulfur than samples taken from non-industrial areas. The sulfur content decreases proportionately to the distance from the source of the smoke. As indicated in Table 1, these features are as pertinent today as before. In samples 1 to 3 the leaves from the portion of the trees facing the smoke contained more sulfur than leaves on the opposite sides. In samples 4 and 5 the recorded S02 values decreased with increasing distance from the factories. Since the above-described phenomenon recurred year after year, S02 was held responsible for the damage.

During the past 10 years F- compounds were found to share in the damage which had been formerly attributed solely to S02 (4,5). In horticultural operations in the Netherlands F- compounds were recognized as the major cause of damage (6). Finally in 1962, Bohne (7) reported that gaseous products of combustion containing F- compounds caused most of the damage in horticulture, farming and forestry. The following observations further support this concept:

Situation I

After conversion of a brickworks from an annular kiln to a tunnel kiln and, along with it, to the use of oil as fuel, severe damage appeared on leaves of many trees in the surrounding gardens. Because of the clearly visible oily coating on the leaves, the damage in the vicinity of the factory was attributed to oil and to S02 liberated during combustion. Severe, sharply defined areas of necrosis were noted on the margin of otherwise bright green leaves. Depending on the plant species, the necrotic lesions on the leaf margins exhibited different shades of brown, which were typical of F- damage. In gladiolus plants, red-brown areas beginning at the tips and slanting to the central portion of the leaves were evident. The leaves on the portion of the trees and bushes facing the factory showed much greater damage than on the opposite sides. Chemical analyses of the washed parts of the plant are presented in Table 2. Fluoride assays were carried-out according to the method by Gericke and Kurmies (8). None of the samples taken at the end of June showed a high sulfur content, whereas the F- level in all samples was 12 to 23 times above normal (0.5 mg%). Therefore, F- emissions must have been generated by thermal decomposition of the F-compounds contained in the clay during the firing of the brick.

Situation Il

In another community the circumstances were more complex. Farmers noted severe damage to vegetation. Beef cattle grazing on pasture became emaciated and/or died after a short illness. Residents complained of headaches and throat irritation during the first few weeks after they had moved into the area. Emissions from three neighboring factories, an electric power station, a briquette factory, and an iron-refining smelter were considered the likely cause, and the damage to vegetation was believed to be due to S02. The damage to beef cattle was attributed to accumulation of molybdenum dust from the iron smelter.

Here again, the injury to plants consisted of unusually severe brown-colored necrosis on the leaf margin. The lesions were sharply delineated from the otherwise healthy looking portions of the leaf. Gladiolus plants showed the typical F- injury described above. Gardens near the factories were devastated in strips. Several kinds of cabbage which are intolerant to combustion gases were heavily damaged. The wheat in one field was only 1/2 m high at harvest time and had only small, narrow husks. Table 3 presents the analysis of plant samples.

Sulfur oxides from the air did contaminate the plants, but the concentrations were not high enough to cause the unusually severe damage. With a F- content of over 20 mg% (200 ppm) in the leaves and with 584 mg% of F- (5840 ppm) in wheat straw, F- poisoning of the plants was considered to be the cause of the damage. The effect of F- containing gases on wheat kernels is illustrated in Fig. 1.

Calcium fluoride (CaF2) was used at the smelter as a flux in the process of smelting molybdenum. Since the F- emitting foundry has installed scrubbing equipment for the exhausts, only insignificant damage has appeared in the vicinity of the smelter. Beef cattle is now on pasture in the area without ill-effect.

The typical damage to gladiolus, the brown and sharply defined margin on portions of otherwise green leaves and the greatly increased F- content of the plants pointed to F- rather than S02 injury.

Situation III

Because of the presence of many coal mines, blast furnaces, electric Stations and the high consumption of coal by industry and by the numerous households near large cities in the German Ruhr area, S02 remains a permanent constituent of the polluted air. Forestry workers consider the gas the major factor impeding the cultivation of conifers and hold it responsible for its progressive decline.

In the center of the Ruhr valley, inhabitants complained of heavy damage to numerous trees and flowers. In a large section nearly all cherry trees had died. In another location, pine trees were in a deplorable condition. A nearby power plant which emitted large quantities of S02 was held responsible for the damage. In addition to gladiolus, pears, damson plums, apricots, peaches, birches, horse-chestnuts, currants and peonies were affected. In many areas throughout the whole region emission of concentrated gases of combustion had defoliated certain plants in closely growing groups. Some had already lost their leaves in June and many had died.

Table 4 shows the results of a few assays of numerous others tested for sulfur and fluoride. The sulfur levels varied from 0.82-1.71% indicating an increase due to S02 absorption. However, on the basis of numerous S02 assays of leaves carried out during the past 15 years at sites without S02- emission, S02 cannot be held responsible for the leaf damage. For example, the undamaged parts and the black-brown margins of the pear leaves of sample #5 contained only 1.63% of S03. Such levels of sulphur oxides were not sufficiently high to produce the severe burns at the leaf margins.

On the other hand, the leaves showed unusually high F- levels from repeated exposures to F- containing gases of combustion. Without exception, the black-brown margins of samples 4 to 6 exhibited a much higher F- content than the accompanying green portions of the leaves.

The effect of F- emission on the growth of the above-mentioned pine trees is illustrated in Fig. 2. There had been no disturbance in growth until 1956 after a fertilizer factory, from which gases escaped during the decomposition of raw phosphates, started operation. It was situated between large electric power plants. At that time, the growth in thickness of the pines which had been uniform up to that point, came to an almost complete standstill.

The experience in the three above-described situations indicates that where fluorides and sulfur oxides are emitted simultaneously from industrial complexes, the major portion of the damage is due to F-.

Bibliography

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