LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Using Artificial Soil and Dry-Column Flash Chromatography To Simulate Organic Substance Leaching Process: A Colorful Environmental Chemistry Experiment Isa G. J. de Avellar,* Taís A. P. G. Cotta, and Amarílis de V. Finageiv Neder Instituto de Química, Universidade de Brasília, Campus Darcy Ribeiro, CP 4478, 70904-970, Brasília DF, Brazil
bS Supporting Information ABSTRACT: Soil is an important and complex environmental compartment and soil contamination contributes to the pollution of aquifers and other water basins. A simple and low-cost experiment is described in which the mobility of three organic compounds in an artificial soil is examined using dry-column flash chromatography. The compounds were applied on top of the soil surface, and the column was irrigated with a dilute solution of calcium chloride that mimics rain. The compounds were detected in column drainage fractions using classic qualitative tests in which the intensity of the color produced is related to the concentration of the analyte. The experiment replicates a leaching system in which organic substances migrate through soil at distinct rates as a result of differences in partition constants and water solubility, properties that are related to structural features. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Upper-Division Undergraduate, Environmental Chemistry, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Agricultural Chemistry, Applications of Chemistry, Chromatography
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ew undergraduate courses, such as environmental sciences, environmental engineering, and environmental management, have gained popularity. Most of the new courses focus on the natural processes responsible for environmental equilibrium and examine the substances used in our daily lives that affect the population and the environment.1 Typically, these courses take a multidisciplinary approach, with chemistry playing a significant role. Although environmental issues are usually presented in chemistry courses, many traditional chemistry courses lack laboratory experiments that have been specially designed to simulate the relationships between chemical structure and the behavior of the substance in the environment. Soil is an important and complex environmental compartment and soil contamination contributes to the pollution of aquifers and other water basins. Rain falling on the surface of contaminated soil percolates down into the aquifer, contributing to the pollution of the water. Many different processes are responsible for the behavior of chemicals in soil, among them are macroscopic transport phenomena and molecular interactions.1 The physicalchemical properties of each pollutant, its water solubility, its octanolwater partition constant (KOW), its soilwater distribution coefficient normalized to organic carbon (KOC), and characteristics of the soil itself are fundamental aspects that must be considered when studying the transport of xenobiotic or anthropogenic chemicals in the environment. A simple and low-cost experiment is described in which the mobility of three organic compounds in an artificial soil is examined using dry-column flash chromatography. The compounds, representing different types of pollutants, were applied on top of the adsorbent soil surface, and the column was irrigated Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
with a dilute solution of calcium chloride. This solution, also referred to as artificial rain,2 simulates a soil leaching solution. The compounds were detected in column drainage fractions using classic qualitative tests in which the intensity of the color produced is related to the concentration of the analyte and can be unambiguously identified even when together in the same test tube.
’ POLLUTANTS The compounds, shown in Figure 1, were selected to represent different types of pollutants. 1,10-Phenanthroline is an example of an N-heterocyclic polyaromatic hydrocarbon (NPAH) that is produced during incomplete combustion of organic matter, including wood, waste, and fossil fuels in the presence of nitrogen-containing substances. It is also an industrial byproduct and, similar to other aromatic amines and NPAHs, may be released in the environment from spills, waste, and nontreated effluents from industries that are associated with oil drilling and refining, coal tar processing, chemical production, and wood preservation.3 Phenolphthalein is one of the most widely used acidbase indicators in chemistry laboratories in universities and industries. Although used in dilute solutions, phenolphthalein represents organic substances employed in laboratories globally that are washed down the drain without previous treatment. Salicylic acid is the product of the hydrolysis of acetylsalicylic acid, the active ingredient of aspirin, a worldwide nonprescription Published: August 22, 2011 248
dx.doi.org/10.1021/ed100728j | J. Chem. Educ. 2012, 89, 248–253
Journal of Chemical Education
LABORATORY EXPERIMENT
material, although little information on the toxic effects is available.8 Tests with Salmonella typhimurium are negative for carcinogenesis.9
Figure 1. Structures of the compounds used in the study.
drug mainly used as an analgesic and antipyretic. Salicylic acid and its derivatives are also found in skin-care products. Human pharmaceuticals are a group of emerging and potentially hazardous contaminants that enter the environment mainly through incomplete wastewater treatment of drugs not absorbed by the body. Veterinary pharmaceuticals found in animal waste are also released into the environment directly through infiltration or runoff into surface waters. Additionally, unused pharmaceuticals in general may enter the environment through landfill leachate. Antibiotics, analgesics, psychiatric drugs, and natural and synthetic hormones have been detected in surface water and wastewaters at levels up to a few micrograms per liter (μg L1).4 Pharmaceuticals or their metabolites are biologically active compounds; many are persistent and may bioaccumulate. Furthermore, many pollutants may act as endocrine disruptors, which can potentially affect environmental and human health. More investigation is needed into the fate of pharmaceuticals in the environment, their concentration, their ecotoxicity to particular species that are representative of different environmental compartments, and the possible occurrence of synergetic effects. Salicylic acid is the model compound for this class of pollutants. As wastewater treatment plants are not able to completely eliminate residues of pharmaceutical substances and nontreated industrial and chemistry laboratory effluents, they are released into the environment and can be detected in different environmental samples. Substances that are persistent organic pollutants (POPs), with the potential to be transported to different environmental compartments, are generally detected in all environmental matrices—water, atmosphere, soil, and sediments—sometimes in locations far from the sources of their release. Once any potentially toxic substance is introduced into the environment, it is subjected to several physical, chemical, and biological processes, which will determine its fate in the environment. A complete discussion of the fate of these substances in the environment is beyond the scope of this article, but the release of these three particular compounds into the environment raises concern due to their potential toxicity. Phenolphthalein was an ingredient in some laxative products until 1999 when the FDA reclassified the drug as “not generally recognized as safe and effective” after studies indicated that it presented a potential carcinogenic risk.5 Phenolphthalein is listed in the Eleventh Report on Carcinogens of the National Toxicology Program.6 The same report found it to be genotoxic; that is, it can damage or cause mutations to DNA. The toxic effects on humans of salicylic acid, as well as those of salicylates, are complex; target organs are the central nervous system, lungs, kidneys, and liver. Salicylic acid causes gastrointestinal irritation and is readily absorbed through the skin and may induce toxicity (salicylism).7 1,10-Phenanthroline, as an aromatic amine and also a NPAH, is considered a hazardous
’ SORPTION PROCESS The leaching behavior of organic pollutants in soil is important because it may determine their relative potential to contaminate groundwater resources. Rain and artificial irrigation are the two main processes responsible for the leaching of a pollutant found at the soil surface. The mobility of an organic substance in soil depends on several factors, including its aqueous solubility and its sorption on solid particles present in soil. Sorption is usually the result of more than one type of interaction: it may involve van der Waals (dipoledipole, H-bonding, dispersion) and ionic interactions and covalent chemical bonding between reactive functional groups of the free molecules and specific solid surface sites. When a chemical dissolved in an aqueous solution is in contact with solids similar to those present in soil, a distribution (or partition) will take place between the aqueous phase and the solid phase. At equilibrium, the partitioning process between the sorbate (any particular chemical) and sorbent (any particular solid) can be described by a linear relationship CS ¼ K D Cw where CS (mol kg1) is the total sorbate concentration associated with the sorbent and Cw (mol L1) is the total chemical concentration in the solution. Considering that the whole sorption process occurs at constant temperature, this relationship is often referred to as a sorption isotherm, and KD is known as the soilwater distribution coefficient. KD may vary greatly because sorption depends on the fraction of organic material in the soil matrix; chemical species can also be sorbed by other nonorganic phases of soil, such as clays. As a result, the comparison of KD values obtained from studies using different soil types or even different methods must be done with great caution. Nevertheless, KD is an important tool for estimating the sorption potential of a dissolved pollutant in contact with soil; the greater the KD value, the higher the affinity of a particular chemical for solid particles found in soil or sediments. Because KD is strongly dependent upon the fraction of organic matter in a soil sample, the sorption of a dissolved pollutant can also be viewed as a simpler partitioning process between the aqueous phase and the organic material in the soil, KD ¼ KOC fOC where KOC is the soilwater distribution coefficient normalized to organic carbon and fOC is the fraction of organic carbon in the soil (fOC = kg organic substance/kg dry soil). Because experimental KOC data are not available for all chemicals in use, mathematical correlations have been developed relating KOC to other descriptors, such as aqueous solubility and KOW, the octanolwater partition constant.10,11 KOW refers to a partitioning process in which a compound in contact with two immiscible phases, water and an organic solvent such as n-octanol, mutually saturated with one another, distributes between the organic phase and the aqueous phase until equilibrium is reached. KOW is a dimensionless constant expressed by KOW ¼ CO =CW where CO is the concentration of the compound in the organic phase (n-octanol) and CW is the concentration in the aqueous phase (water).1 249
dx.doi.org/10.1021/ed100728j |J. Chem. Educ. 2012, 89, 248–253
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The liquid phase in soils is a solution composed of water, colloidal material, and dissolved substances, such as ions, organic solutes, and gases, mainly oxygen and carbon dioxide. The composition of the soil solution varies and depends on conditions of temperature, humidity, aeration, and microbial activity.15 In leaching and soil adsorption studies, it is common to use calcium chloride solution in concentrations that may vary from 0.01 to 0.1 g/L to simulate the ionic strength usually found in soil aqueous leaching solutions.18 The concentration of this solution in this range does not seem to present relevant effects in sorption experiments.1,18
Table 1. Tested Chemicals and Properties: Molecular Weight (Mr), Water Solubility (Sw), First Dissociation Constant (pKa), OctanolWater Partition Constant (log KOW), and SoilWater Distribution Coefficient (log KOC or KOC) Chemical Salicylic Acid 1,10-Phenantroline Phenolphthalein
Mr
Sw/(mg L1)
pKaa
log KOW
log KOC or (KOC)
138.12
2240b
2.98
2.26b
(404)b,c
180.21
d
4.84
d
1.54e
b
(490)b,c
318.32
2690
b
400
9.7
1.78 2.41
a
For 1,10-phenanthroline, the value refers to pKa of the conjugated acid.12 b Hazardous Substance Data Bank.7 c Calculated using given KOW. d Data from ref 3. e Estimated value.13
’ EXPERIMENT Explanation and Procedure
The correlation between KOC and KOW is pertinent because the same factors responsible for the distribution of compounds between water-immiscible organic solvents and water also govern environmental partitioning from water into natural organic matter of soil or sediments. However, these correlations are suitable for specific groups of chemicals and may vary between compound classes. Table 1 lists reported values for water solubility, pKa, KOW, and KOC of the tested compounds. The KD value was found in the literature only for salicylic acid and with a broad variability depending on the soil sample characteristics.
The dry-column flash chromatography procedure described in this Journal19 was employed to evaluate mobility of organic compounds in an artificial soil. The experiment aims to replicate a leaching system in which organic substances migrate through soil as a result of differences in KOW, KOC, and water solubility, properties that are related to their structural features (Table 1). This experiment is intended for chemistry students with knowledge of chromatography principles and can be used in a general or organic course. For students from environmental sciences and environmental engineering courses, the theoretical content presented in the introduction is adequate to understand the phenomena of sorption and partition underlying the experiment. The experiment can be accomplished in 3.5 h with the students working in groups of two.
’ SOIL CHEMISTRY The mobility or the retention of a pollutant in soil is determined by the efficiency of sorption processes, which are governed by the chemical and physical properties of the soils and chemicals. Soil is a variable mixture of minerals, organic matter, water, and air and is strongly dependent on its origin and history. It can differ in particle size distribution, arrangement, and compaction, in the quantity and type of organic matter, and in the mineralogy of the inorganic particles. Soils may be characterized by their inorganic particle size: clay (
