Where Soil Meets Dirt
“What is Muck” via HMGA Water Project
In these days of COVID-19, we are hearing a lot about bleach.
The Clorox Company makes a much sought after product — bleach germicidal disinfectant wipes — premoistened sheets with a solution of 0.55% sodium hypochlorite as the active ingredient; this represents an approximately 1:10 dilution of typical household bleach.
Household chlorine bleach is a powerful laundry additive for the oxidation of body grime and food stains on clothing.
I say powerful as both a descriptor of its cleaning action, and as a warning to users of the product.
The manufacturers of chlorine bleach have designed varieties that are less likely to splash, and have included hazard labeling and website warnings such as:
Harmful if absorbed through skin.
Do not get in eyes, on skin, or on clothing.
Harmful if swallowed.
Wear protective eyewear (goggles, face shield or safety glasses).
Bleach and other disinfectants are not suitable for consumption or injection under any circumstances.
Bare-hands contact with full-strength bleach is a dangerous practice.
For those who have done this by accident, the sensation is strange — one experiences a slippery sensation between the fingertips; that is the feel of the outermost layer of skin dissolving.
Typical household bleach is a highly alkaline (~pH 12) mixture of sodium hypochlorite (NaOCl; concentration in aqueous solution ranging from about 2-10%) and sodium hydroxide (NaOH), the latter added to raise the solution pH.
Bleach is not the only oxidizing agent used in homes, and indeed the term bleaching extends beyond sodium hypochlorite.
Bleaching of hair in the dyeing process refers to the use of hydrogen peroxide (H2O2) as the developer, to oxidize, and thereby decolorize, the natural pigments (melanin) in the hair shafts.
Depending upon the process chosen, the concentration of hydrogen peroxide may be from 3% (typical drugstore disinfectant strength) to 12%.
So bleach and hydrogen peroxide are things that we use in our everyday lives for removal of unwanted organic materials.
The strength of these solutions is chosen to achieve the desired effect (for example, the removal of a pizza stain on a tee shirt) without the complete destruction of the underlying cotton fabric.
Well, in soil science, we often face a similar challenge — for example, the removal of soil organic matter (OM) that binds mineral particles together in order to determine the particle size distribution (the % of sand, silt and clay), or the identity of the clay minerals present.
Such analyses are critical to the characterization of soil properties.
As in the case of the tee shirt, we want the removal to be selective — digesting the OM but leaving the mineral components intact.
For a long time, 30% hydrogen peroxide — about 2.5 times the concentration used to strip hair shafts of their melanin — was the reagent of choice for destruction of soil OM, and the reaction was enhanced by raising the digestion temperature to about 60 - 90 ºC.
But concerns remained about the attack of concentrated, hot hydrogen peroxide on the underlying mineral components, and additionally, this concentrated peroxide reagent was expensive.
So around 1960, investigators with the Oklahoma Geological Survey, seeking to disaggregate black shales (i.e., shales containing abundant OM) at a reasonable cost, pioneered the use of Clorox bleach as an active agent to remove the cementing OM, and thus serve as a pretreatment for clay mineral analysis.
The idea for using Clorox in these geological studies came from contemporary investigators in allied fields where the bleach had been successfully employed to digest the soft tissues of shelled invertebrates and pollen.
This sharing of methods across disciplinary boundaries is characteristic of the earth sciences, and in this way, soil scientists took note of these black shale studies.
The selective nature of extracting solutions is fundamental to the field of soil science — uniquely chosen extracting solutions are used to assess soil properties such as:
the quantity and degree of crystallinity of iron and manganese coatings on soil particles (more on this in a future blog post…),
the fraction of contaminants occupying exchangeable positions on clay minerals (and thus available for transfer to percolating water), and
the fraction of metals bound to soil OM (and thus acting to inhibit the decomposition of the organic molecules, thereby promoting carbon sequestration).
Additionally, what commonly fall under the umbrella of soil testing, are a group of selective extraction procedures and chemical analyses, adapted to large-scale usage to meet the demands of gardeners and farmers, that commonly assess levels of plant-available nitrogen, phosphorus and potassium (the “NPK” of fertilizer formulations), and may be expanded to include sulfur and micronutrients such as zinc and boron.
The development of such tests is largely empirical, and modifications of extractant type are put in place to adjust for regional differences in soils
The Oklahoma geologists had made it abundantly clear in 1960 that Clorox was a highly aggressive chemical agent, noting that attempts to use a 3-day, 70 ºC digestion of their shale samples also resulted in the dissolution of the glass beakers!
Soil chemists, interested in a milder, more selective removal of OM, moved to the use of reagent-grade (higher purity variants of chlorine bleach, used in analytical chemistry) 5% sodium hypochlorite alone (without the sodium hydroxide present in Clorox), adjusted to pH’s around 8.5 to 9.5 by the addition of hydrochloric acid.
Such solutions were used as pretreatments for mineralogical analysis, and in studies of soil contaminants such as cadmium.
These latter treatments are often used in multistep, sequential extractions, done in centrifuge tubes, to identify the host components of metal contaminants in the soil.
When it comes to the sodium hypochlorite step used to quantify the OM-bound metals, care must be exercised for the sake of safety and the elimination of sample loss; thus the caps of the tubes are loosened to allow for the escape of carbon dioxide gas produced by the oxidation of OM.
A recent, 30-second ad by the New York Times, titled The truth is essential, proclaims “Bleach is not a cure.”
This can also be said for the complex problem of soil OM digestion.
But from a historical perspective, it is instructive to note that laundry chlorine bleach paved the way for the development of an improved, highly selective extraction methodology.
For additional reading:
Zoe Ann S. Ahnstrom and David R. Parker (1999) Development and assessment of a sequential extraction procedure for the fractionation of soil cadmium: Soil Science Society of America Journal, v. 63, p. 1650–1658.
Elisabeth Anderson and Jinpeng Li (2020) COVID-19 – Disinfecting with Bleach, Center for Research on Ingredient Safety, Michigan State University, available at https://www.canr.msu.edu/news/covid-19-disinfecting-with-bleach ; accessed May 9, 2020.
James Ubbe Anderson (1963) An improved pretreatment for mineralogical analysis of samples containing organic matter: Clays and Clay Minerals, v. 10, p. 380-388.
Martin M. Cassidy and Charles J. Mankin (1960) Chlorox [sic] used in preparation of black shale for clay mineral analysis: Oklahoma Geology Notes, v. 20, p. 275-281.
Clorox Professional Products Company (2017) Technical Information Clorox Healthcare® Bleach Germicidal Wipes, available at https://www.cloroxpro.com/resource-center/clorox-healthcare-bleach-germicidal-disinfectant-wipes-technical-info/ ; accessed May 9, 2020.
Anne Marie Helmenstine, Ph.D. (2020) The Science of Hair Coloring, available at https://www.thoughtco.com/salon-hair-color-chemistry-602183 ; accessed May 9, 2020.
Maffew James (2019) How to Bleach Hair, available at https://bellatory.com/hair/How-to-bleach-hair ; accessed May 9, 2020.
North Central Regional Committee for Soil Testing and Plant Analysis [NCERA-13] (2015) Recommended chemical soil test procedures for the North Central Region, North Central Regional Research Publication No. 221 (Revised August 2015), Missouri Agricultural Experiment Station SB 1001, available at https://extension2.missouri.edu/sb1001 ; accessed May 13, 2020.
Avraham Saig (2012) Why does my skin turn smooth after I touch household bleach?: Davidson Institute of Science Education, Weizmann Institute of Science Education, Rehovot, Israel, available at https://davidson.weizmann.ac.il/en/online/askexpert/chemistry/why-does-my-skin-turn-smooth-after-i-touch-household-bleach-orit-0 ; accessed May 9, 2020.
