Determination of Mercury Uptake in the Common Green Bean, Phaseolus vulgaris by ColdVapor AAS and Sc

1
Determination of Mercury Uptake in the Common
Green Bean, Phaseolus vulgaris by Cold-Vapor AAS
and Scanning Electron Microscopy.
Abstract Plants have the capability of absorbing
toxic metals such as mercury. These plants may be
consumed by humans. Exposure to high
concentrations of mercury usually results in
degenerative neurological effects in humans.
Green beans, Phaseolus vulgaris, have been shown
to be bioaccumulators of heavy metals, including
mercury, through the soil from air deposition of
pollutants. This study was conducted to determine
the various Hg concentrations within the
different tissues of the plant and any
morphological changes that occurred with these
concentrations. The plants were exposed to
varying mercury concentrations in the soil and
the lowest observed effect concentration was
identified that displayed negative structural
effects. Using CVAAS, mercury was detected in
every tissue of all the sample groups in the
plants.The group that contained the most mercury
in the beans was 0.04 mg/L, which is below the US
EPA health limit. SEM micrographs display various
structural effects such as the visible absence of
fine root hairs on the secondary root structure
and a reduced number of stomata on the surface of
the leaf at 1.5 mg/L of mercury in the soil.

• Kathleen M. Wilson and David Orvos
• Environmental Studies Department
• Sweet Briar College
• Sweet Briar, VA 24595

A 1.5 mg/L leaf
B Control leaf
C1.5 mg/L root
A
D Control root
F Control root magnified
E 1.5 mg/L root magnified
B
C
Figure 3 SEM photomicrographs of plant tissue.
Photos B,D, and F are of the control plant with
no Hg. Photos A,C and E are of the 1.47mg Hg
plant. Note the difference of less fine root
hairs in Photos C and D and the decrease of
stomata in Photos A and B. Photos E and F are
magnifications.

• Method Design
• Seeds of Phaseolus vulgaris were planted
  approximately 3 cm deep in Peat Pots containing
  54 g organic potting soil. The plants were grown
  in an incubator at 25C set to 1410 light
  dark cycle to mimic early summer conditions. Upon
  germination to a height of approximately 1.0 cm
  above the soil, the plants were dosed with water
  containing mercuric chloride of 0.5, 1.0, 2.0 and
  5.0 mg/L. The duration of the growing period was
  twenty-five days.
• SEM Sample Preparation
• Five samples were prepared for each bean plant.
  They were fixed in formalin overnight before
  undergoing a five step graded ethanol series
  followed by sublimation of Peldri. The specimens
  were mounted on stubs using carbon double-sided
  tape. They were sputter coated in gold-platinum
  before being analyzed under the SEM.
• AAS Analysis
• The plants were divided into the following
  components leaves, beans, stems, roots and
  soil. Each of these samples were weighed, dried,
  then reweighed prior to acid digestion. EPA
  method 3050B was used for acid digestion. Water
  eluent samples from the plants were also included
  in the analysis. The samples were analyzed on a
  PerkinElmer Analyst 800 using cold vapor and
  sodium borohydride as the reductant.

E
D

Figure 1 The PerkinElmer Analyst 800 instrument
used in this research.
Background The primary source of mercury
contamination in the environment is air
pollution. Mercury content in coal for the United
States ranges between 0.070 33 part per
million (ppm). As the coal is burned, the
mercury volatizes into the atmosphere, where it
can then be deposited on vegetation or the soil.
The Environmental Protection Agency (EPA) found
the average concentration of mercury in US soil
to be less than 0.2 mg/Kg. If the growing
conditions of a plant are altered, such as the
increase in mercury concentration in the soil,
the plant may respond with abnormal growth
conditions. At high concentrations, mercury
may cause chlorosis, necrosis, abscess of older
leaves, and overall growth reduction. The roots
serve as the primary site of mercury uptake in
the plant. Root hairs are extended outgrowths of
epidermal cells and provide a large surface area
of contact with the soil, serving as the main
source of absorption of both water and nutrients.
The common green bean, Phaseolus vulgaris, is a
dicotyledonous plant which Research has shown to
be sensitive toward mercury. Due to its
sensitivity, it was used for this study.
 
Figure 3 represents the detected Hg in the
various plant tissues for each of the treatment
group of plants. Figure A represents the blank
treatment group the with the following figures of
B,C,D, and E representing the same data for the
other Hg treatment groups. Standard deviation is
shown by the error bars on the graphs.
Results In this analysis, the highest mercury
concentration for the 3.7 mg/L plants was fount
in the soil to be 16.86 mg/Kg. For the 1.5 mg/L
treated plants, the highest concentration was
also in the soil to be 12.42 mg/Kg. The 0.74 mg/L
plants had the highest concentration of mercury
in their leaves at 22.06 mg/Kg. The 0.04 mg/L
plants had the highest mercury concentration
detected in their leaves at 10.86 mg/Kg.
Chlorosis and necrosis was evident in the 0.74
mg/L, 1.5 mg/L and 3.7 mg/L plants. These effects
may be contributed to the decline in the health
of the plant and the low mercury concentrations
detected in the leaves.

• Conclusion/Implication
• This research has determined that mercury causes
  morphological changes in a plant that include
  reduced number of stomata per leaf and the
  absence of fine root hairs. Using the SEM, the
  lowest observed effect concentration (LOEC) was
  1.5 mg/L. In the CVAAS analysis, mercury was
  detected in every tissue sample from all the
  treatment plant groups. The LOEC from this
  analysis was the 0.74 mg/L treatment group, due
  to observable chlorotic effects. Both the LOEC
  concentration determined from the SEM micrographs
  and the CVAAS LOEC are above the EPAs average
  concentration found in top soils across the
  United States. If the rate of mercury released
  into the atmosphere continues to increase, this
  may result in detrimental global agricultural
  implications.