Category: Environment

Bryophytes : Seeking The Root(less) Origins of Mud

Bryophytes : Seeking The Root(less) Origins of Mud

Reading Time: 9 minutes

For 90% of its existence, planet Earth (mostly) lacked the one thing that cradled civilizations and made millions for detergent companies : mud. Then, sometime around 458 million years ago, it suddenly began seeing a lot more of Mr Mud. Why the sudden rise? How the sudden rise? What in the sodden rice is mud, anyway? And where exactly do those rootless plants called  bryophytes fit in? The Nerd Druid investigates the root(less) cause behind this muddy mystery.

India and farmers

It is March, spring in the Northern Hemisphere. In East India, along the hot dusty flyovers of Kolkata, crimson Rudropolash flowers bloom, giving the speedway a red bannister. Thousands of kilometers away, tens of thousands of impoverished farmers march 180 km barefoot from Nashik to Mumbai to make the government hear their plight. Their red headdress contrast and clash with the saffron of the Indian summer like a tide of Rudropolash, reminding ideologies and ideologues that governments are, ultimately, of, for, and by the people. It is time for the Indian spring [Note:IndianSpring].

Image of impoverished Indian farmers marching 180 km from Nashik to Mumbai to voice their grievances. Their red turbans, headdresses, and flags give the illusion of red Rudropolash flower in full bloom.
Indian Spring

This is not a political blog. However, science and politics sometimes seem rather (unhappily) closely entwined. In this particular article, we shall have no opportunity to explore said entwinement.

A very brief history of farming

Instead, we come to the basic reason why farmers are. Given arable land, farmers grow food. Without farming and agriculture, civilisation as we know it (and as we clearly are very not aware of) would not have existed. Primitive hunter-gatherers, perhaps exhausted with having to chase after or pluck food all day, decided, about ten thousand years ago, that enough was enough. Here be river. Here be fertile land. Here be seed. Where be hammock?

And that is how modern civilisation arose. Through extremely sturdy king-sized hammocks.

Of course, just like you can’t make fire without fuel, or babies without incredibly strong storks, you need a proper substrate for good crops to grow. Luckily, riverbeds and its frequently flooded adjoining areas provided such substrates. The Indo-Gangetic plains in India, Pakistan and Bangladesh, and the North China plains are one of the largest such alluvial plains, as also one of the most fertile in the world.

Map of the Indo-Gangetic Plains. A large percentage of the world's alluvium deposits are found here.
The Indo-Gangetic Plains


Alluvium (from the Latin alluvius, from alluere, “to wash against”) is loose soil or sediments that have been eroded and deposited by water on land, and which has not yet compacted into rock. Alluvium comprises fine particles of silt and clay, as also larger particles of sand and gravel. Agriculturally, alluvium is gold. The high mineral content, optimal water-retention capacity, and optimal breathing room for plant roots make it ideal for a successful harvest. Unsurprising then, that a third of the world’s population now lives in the two largest alluvial plains I spoke of a minute ago.

Geologically, alluvium is quite young, with most deposits in the world having formed in the Quaternary Period of the Cenozoic Era.

Wait. The what period of the what era?

Earth’s geological eras

Well, Earth’s geological timeline is divided into several time-sections, for lack of a better word. The largest sections are called Eons. There are four of them, though for all intents and purposes, especially for the intent and purpose of this article, we’ll talk about two : the Precambrian [Note:Precambrian], and the Phanerozoic. The boundary between the two, situated 542 millions years in the past, is approximately when complex life (such as trilobites or corals) began appearing on Earth. The name phanerozoic, from the Ancient Greek words φανερός (phanerós) meaning visible and ζωή (zōḗ) meaning life, is quite apt.

A cartoon schematic of Earth’s geological timeline, designed by Ray Troll. It shows the geological ages stacked up like a pyramid.
Earth’s geological timeline, in cartoon version. Designed by Ray Troll.

Actually, I lied. I’m not going to talk about the Precambrian, not in this article anyway.

Back to the classification systems. Eons are divided into Eras; the Phanerozoic Eon has three Eras — the Paleozoic (paleo = old, ancient), the Mesozoic (meso = middle), and the Cenozoic (ceno = recent, new). We live in the Cenozoic Era, in the Quaternary Period, in the Holocene Epoch. Epochs can be further subdivided into Ages, though the Holocene hasn’t been long enough to admit such a division.

Eons > Eras > Periods > Epochs > Ages

There. That should clear things up.

The Paleozoic Era

We are, today, more interested in the Paleozoic Era. Beginning 542 mya (mya = million years ago) with the appearance of complex life, the Paleozoic Era ends 251 mya and gives way to the Triassic Period of the Mesozoic Era.

Hold on. Triassic? Does that mean the Mesozoic Era…?

Yep. The Mesozoic Era has three Periods : Triassic (251 – 200 mya), Jurassic (200 – 145.5 mya), and the Cretaceous (145.5 – 65.5 mya). The Mesozoic is the “Age of the Dinosaurs”!

Which (a) makes no sense, it should have been the “Era of the Dinosaurs” and (b) makes no sense, because we ain’t talking about dinos today. Curse you Michael Crichton [Note:Crichton].

Rewind. Back to the Paleozoic.

Schematic of Earth’s geological timeline, in all its detailed glory. This is the International Chronostratigraphic chart, updated February 2017.
Earth’s geological timeline, in all its detailed glory/

Incorporating 291 million years of Earth history, the Paleozoic Era is divided into six Periods : Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Cambrian saw the greatest number of species evolve in a single Period, in an evolutionary event called the Cambrian Explosion. The animals, or rather, the fauna of this Period was dominated by the hard-shelled trilobites. Fish, coral and cephalopods (octopuses and squids, among others) evolved in the Ordovician, though fauna were dominated by trilobites, snails and shellfish. The Cambrian Explosion was balanced out by the Ordovician-Silurian extinction event, where almost half of all marine life disappeared. Things warmed up in the Silurian Period, which saw an explosion in the evolution of fish (both jawed and jawless), as well as the rise of vascular plants on land.

Vascular plants

Most plants and trees that we see today have a highly evolved, though simply explained, mechanism of life. Roots burrow deep into the soil, providing support and extracting water and minerals. These raw materials are then transported throughout the plant body via the rigid xylem tissues. Leaves “cook” these raw materials using sunlight to photosynthesize “plant food”, which is then transported via the less rigid phloem tissues to other parts of the plant body. Plants that have the xylem-phloem (XP) transport structure are vascular plants. The earliest such plants, Cooksonia being a wonderful representative, were the earliest precursors to such XP plants. Nowadays, ferns, conifers, and angiosperms (flowering plants) are all vascular, and all have XP.

Photographs showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips
Photographs showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips

Which shouldn’t be surprising. All those millions of years gives one lots of experience [Note:XP].

Roots, of course, do more than support and feed the plant. In rocky terrain, they sneak into crevices and break the rock apart into stones and pebbles, enabling wind and water and sun to break them further into finer particles of sand, silt, and clay. Thus, soil! In plain land, or mountain slopes, they hold onto the topsoil, preventing rainwater and wind from blowing it away. Roots are thus both friend and foe to Sir Erosion, and are quite critical for Sirs Environment and Ecology.

Classifying soil and rocks

Since we are on a classification spree today, would we like to try and classify rocks, pebbles, and soil based on how large or tiny the particles are? Bet we would. The Wentworth scale will help immensely.

Image of the Wentworth scale of classifying soil particles
The Wentworth scale of classifying soil particles

Anything above 256 mm (diameter) are boulders. 64 – 256 mm are cobbles, 2 – 64 mm are gravel (4 – 64 mm are pebbles), 62.5 µm – 2 mm is sand, 3.9 – 62.5 µm is silt, and 0.98 – 3.9 µm is clay [Note:Wentworth]. Silt and clay together make up what is usually called mud.

That takes care of one question. Two more to go.

The rise of mud is a bit of a chicken-and-egg problem. Rooted plants break apart rocks which are then eroded into silt and clay; thus, mud. Therefore, rooted plants must have come first, right? However, rooted plants also need mud for the roots to carry out their primary functions. You can’t leach water and minerals from hard rock, can you, Mr Joe Root [Note:JoeRoot]?

Houston, we seem to have a problem.


Which is where bryophytes come in.

The term bryophyte comes from Greek βρύον (bryon), meaning tree-moss or oyster-green and φυτόν (phyton), meaning plant. Bryophytes are land plants and include various species of liverworts, hornworts, and mosses. Bryophytes arose in the Ordovician Period, sometime around 470 – 460 mya. Around 458 mya, they began to proliferate.

Image of a bryophyte. This is Marchantia, an example of a liverwort.
Bryophyte. This is Marchantia, an example of a liverwort.


But was there mud at that time? Yes, mud was present at that time, mostly in ancient river deposits. This mud originated from the action of non-biotic erosional agents (wind, rain, water, sun) as well as biotic ones (microbes and fungi). However, the percentage of mud (silt+clay, remember?) with respect to the coarser and larger sand and gravel was woefully small.

How small?

William McMahon and Neil Davies, both geologists from the University of Cambridge, decided to find out. They trawled through nearly 1200 published papers for data on mud rock in 704 ancient river deposits. As if that wasn’t enough, they themselves collected new data from 125 ancient river outcrops. Then they calculated, based on relative thickness of mud layers and sand+gravel layers, the percentage of mud (PoM), 458 mya. Their median result?


Woeful doesn’t even begin to describe it.

However, McMahon and Davies weren’t done yet. They went ahead and calculated the median PoM of times after 458 mya. And they found that, over the next 100 million years or so, the PoM kept on increasing until, at about the end of the Devonian Period (359 mya), the PoM had reached 26%!

This was, of course, good news for rooted vasculars, which arrived in mid-Siluria around 430 mya. Given mud to grow on, rooted vasculars would be further able to make more mud. And the chain was established.

Except, what happened between 458 – 430 mya? How did the PoM increase from 1% to about 10% in the intervening 28 millions years? Do we have someone to blame?

Graph showing the percentage of mud and the proliferation of microbes and plants in the Paleozoic Era. Bryophytes start thriving around mid-Ordovician, about 470 mya. The percentage of mud begins to rise from late Ordovician and early Silurian, about 460-440 mya.
Percentage of mud and the proliferation of microbes and plants in the Paleozoic Era

Bryophytes and the origin of mud

I can see the tiny bryophytic child jumping about at the end of the classroom, raising her hand and trying desperately to say,

“It was me! It was me! I made mud.”

To which the geology teacher would inevitably ask, “How?”

Well, to start with, even tiny scrappy little green mats are, in large enough numbers, enough to stop wind and water wash away existing mud into the river and then the sea, ensuring enough mud stays on riverbanks. By stabilising riverbanks such, they might have even altered the paths of rivers and streams, thus changing the landscape towards a state conducive to more plant growth. Finally, organic acids secreted by the bryophytes during photosynthesis would have altered soil chemistry and led to the actual creation of mud.

So, basically, to recap, bryophytes (liverworts, hornworts, and mosses)

  1. Made mud
  2. Made sure the mud stayed
  3. Which led to the rise of the rooted plants
  4. Which led to more mud
  5. And more rooted plants
  6. And thus, agriculture!

Moss are the Boss!

Image of the evolution of plants. Bryophytes such as mosses, liverworts, and hornworts feature prominently.
The evolution of plants



Note:IndianSpring : Or not. As of today, March 13 2018, the Maharashtra government, which was inclined to acquiesce to the farmers’ demands, has indeed acceded to said demands, and have arranged for special trains to put the farmers back in their places.

Note:Precambrian : Well, technically, the Precambrian is a super-Eon, further divisible into three Eons : Hadean, Archaen, and Proterozoic. However, as papa Feynman used to say, what’s the point of learnin’ names if ya don’t know what they do?

Note:Crichton : Actually, no, bless you. You, sir, single-handedly made people fall in love with the prehistoric (more precisely, the mesozoic). Dinosaurs became cool, thanks to you. Scary, too. As The Nerd Druidess loves singing (in perfect rhyme and rhythm)

Oh Michael Crichton

Your dinosaurs

They so wonderfully frighten

Note:XP : XP, in roleplaying game (rpg) lingo, means Experience Points. Given that Dungeons and Dragons was one of the first popular rpgs, it is entirely possible Gary Gygax came up with the name. I wonder how many XP a Nerd Druid class might accumulate.

Note:Wentworth :  Humans speak in decimal. That is, (most of) our activities are carried out in powers of 10. We have ten fingers (101). A hundred meter (102) race is the most exciting, now that Bolt has retired. A thousand kilos (103) makes a ton(ne), though cricket commentators seem to think a century is a ton. Blame the Brits for that. Agriculture arose ten thousand years ago (104), a hundred thousand (105) is called a lakh in India, while a million (106) is probably the most used word in this article.

Computers, however, speak in binary (or octal, or hexadecimal, but never mind). Powers of 2. The method of classification of grain sizes, that is, the Wentworth Scale, also uses powers of 2. Rewriting the paragraph in the main text in terms of powers of 2 and millimeters, we have

Anything above 28 mm (diameter) are boulders. 26 – 28 mm are cobbles, 21 – 26 mm are gravel (22 – 28 mm are pebbles), 2-4 – 21 mm is sand, 2-8 – 2-4 mm is silt, and 2-10 – 2-8 mm is clay.

Binary rules!

Note:JoeRoot : Absolutely no intention of evoking the fine English batter here. Although, truth be told, Root is definitely no average Joe.


  1. McMahon, William J. and Davies, Neil S. : Evolution of alluvial mudrock forced by early land plants, Science (2018) [doi : 10.1126/science.aan4660]
  2. Fischer, Woodward W. : Early plants and the rise of mud, Science Commentary (2018) [doi : 10.1126/science.aas9886]
  3. Yirka, Bob : Ancient rootless plants linked to increase in production of mud rock, (2018)
  4. Gramling, Caroline : Early land plants led to the rise of mud, ScienceNews (2018)


Cleaning up our waters : Graphene microbots to the rescue?

Cleaning up our waters : Graphene microbots to the rescue?

Reading Time: 15 minutes

Water. Without it, life would not begin on Earth. Without it, life could not continue on Earth. With each passing day, effluent from the modern ultra-industrialised world is making sure that the little water available to us is being effectively rendered unusable. Chief among such pollutants are heavy metals such as arsenic and lead, the large-scale removal of which is a protracted and expensive job. Recently, researchers at the Max Planck Institute for Intelligent Systems have designed graphene-based motile micromachines that zip around contaminated water on their own, scoop up lead, and can be guided to a safe place for removal of lead. What’s more, these Wall-E-Bots can them immediately be reused. The Nerd Druid takes a closer look at these microscopic marvels.



Ever since Henry Ford set up his famous assembly line and started belting out Model Ts every two hours or so, the speed at which the manufacturing industry operates has gone up by leaps and bounds. In the past fifty years, this rate has been supercharged with the introduction of robots and automation. The development of new technologies and faster production rates have led to progress and an increase in quality of life. One of the primary such industries is microelectronics, without which this article would probably not have been written1. An iPhone, one of the most advanced machines ever built by humans2 can now be put together in a matter of hours, if not minutes. However, appliances such as laptops and smartphones would be useless without batteries, the fabrication of which would require the chemical technique of electroplating. All of this needs raw materials which the mining industry takes care of, supplying metals such as iron, copper, lead, and metalloids such as silicon and arsenic. Of these, lead and arsenic are toxic and easily contaminate water since they aren’t easy to remove on a large scale. So it quite fortunate that unused raw materials and waste leftovers from these enterprising industries are, instead of being unceremoniously dumped (to use a technical term), disposed off carefully and with due consideration to the environment, taking care not to contaminate water that might be used for drinking or agriculture or human or ecological activities that would be in dire straits should the water be contaminated. Quite fortunate, indeed, non¿

Quick aside : “¿” is the percontation point, also known as the irony punctuation. Taking the form of a reverse question mark, it is used to denote sarcasm in writing. In daily life, one resorts to the Hofstadter Method3.


Heavy metal toxicity

Toxic heavy metals such as lead, mercury and arsenic4 are often causes of environmental contamination and pollution, especially of the soil and water. Acute exposures to any of them causes nausea, vomiting, diarrhoea and/or brain dysfunctions. Chronic exposures can cause cancer, abnormal or de-pigmentation and/or hardening of skin, kidney malfunctions, and diabetes.

Arsenic poisoning

The adverse effects of these chemicals were known from ancient times. Napoleon is thought to have died (killed?) from arsenic poisoning1Groundwater contaminated by arsenic is a major issue in Bangladesh and the eastern part of India, especially in the state of West Bengal.

Mercury poisoning

Mercury was often used in the manufacture of felt hats in the 18th and 19th centuries, and was responsible for a peculiar sort of madness, now diagnosed as erethism, seen in hatmakers of that era, and possible inspired a certain Lewis Carroll to dream up the Mad Hatter. Qin Shi Huang, the first emperor of unified China and the founder of the Qin dynasty, whose name literally means “First Emperor of China”, died of mercury poisoning2, prescribed to him in hopes of granting him eternal life. A wise attempt, non

Too much irony(ing) can make your article sound flat — Ancient blogospheric proverb.

Lead poisoning

Lead has been around a long long time. It was one of the metals known in antiquity, and, for a long time, was prized due to its low cost and softness, which enabled it to be worked easily. However, its toxic effects have often popped up in history.

The Devon Colic

In the 17th and 18th century, the English county of Devon experienced chronic and occasionally fatal colic pains. Symptoms of the Devon colic included muscle aches and terrible pains in the bones. Paralysis was next, followed by death in the most severe cases. While the cause eluded initial attempts, it was soon found to be related to the drinking of cider, an alcoholic drink fermented from apples. Devonians considered the drinking of cider to be traditional, and ascribed the connection of the colic and the cider to a perceived increase in the acidity of the drink. More than a century passed before a Dr George Baker proved, in the 1760s, that the presence of lead in the apple presses and in the lead shots used to clean the pressed juice contaminated the cider and caused the colic. In an ironic rehearsal of the 20th century campaign by tobacco companies to disassociate smoking from lung cancer, the cider industry of Devon carried out a largely ineffectual campaign to disprove Baker’s results. One cider maker, a cleric named Thomas Alcock, wrote a rebuttal to Baker titled

The endemial colic of Devonshire not caused by a solution of lead in the cyder.

“The endemial colic of Devonshire not caused by a solution of lead in the cyder”, by Thomas Alcock. Alcock was a cleric and a cider maker who wished to rebutt the claims of Baker that the Devon colic was caused by lead poisoning from drinking “cyder” (caption reproduced from wikipedia).

Alcock, along with the rest of his ilk, failed to stop whatever they were looking to stop. Which of course meant that, by 1818, the Devon colic was history.

Roman aqueducts

Vitruvius, engineer to Julius Caesar, is remembered today mostly indirectly, through the famous anatomical drawing by Leonardo da Vinci, who was inspired by the former’s observations and analysis of human anatomy and proportions. Vitruvius noticed that Romans were getting a little addled in the head, and traced the problems to the use of lead in the famous aqueducts of Rome. He wrote

“water is much more wholesome from earthenware pipes than from lead pipes. For it seems to be made injurious by lead, because white lead is produced by it, and this is said to be harmful to the human body.”

The Flint Water Crisis

Closer in time, the Flint water crisis in 2014 demonstrated precisely the light years of progress humanity has made in the two millennia since the ancient engineer. Flint is a small city in the US state of Michigan. Before 2014, Flint would source its drinking water from Lake Huron5, and the Detroit river. In this time, the neighbouring Flint River was used as backup supply. In 2014, due to financial and other complicated reasons, the city of Flint decided to shift away from Lake Huron and the Detroit River, making Flint River the only source of drinking water. This happened on April 21. Pretty soon, people began to complain about the colour, taste and odour of the water. Early next year, people started complaining about health issues. Scientists found highly elevated of lead in the Flint drinking water, as well as elevated blood-levels of lead in children, and declared the water unsafe for human consumption. The EPA wrote a memo highlighting the sorry state of affairs at Flint, in response to which a Michigan Department of Environmental Quality official declared on public radio that

Anyone who is concerned about lead in the drinking water in Flint can relax.”

In late December, the city, and a month later the entire county, declared a state of emergency. Luckily, on October 16, the Flint River supply was cut off and the Huron water supply restored. On May 4 2016, President Obama visited the city and drank a glass of water. He did not say, “May the Force be with you”. Nor did he wish that the Force be with him. Or at least away from a certain orange toupeed nincompoop.

Lead used to be a metal of choice for water pipes, both for its cheapness and ease of use. Between 1901 and 1920, water pipes installed in Flint were made of lead. However, although lead from pipes do leach into water flowing through it, the levels found in Flint were higher than normal. Also, lead poisoning wasn’t the only problem that affected Flint post-switch. One of the truck assembly plants belonging to General Motors shut shop and went away. Workers at the plant had begun to notice rust spots on newly manufactured parts. The reason was attributed to corrosion due to chlorides. But while the Flint River has had a history of decades of pollution from industrial effluents, where were the chlorides coming from?

Gladyes Williamson holds up a discolored jug of water and chants along with other protestors outside the Farmers Market downtown on April 25, 2015. (caption from source)

Flint is a part of Michigan, a northerly state in the US. In winters, Flint and its neighbouring areas see moderate snowfall and occasional frost. Ice tends to form on roads, making driving a dangerous activity. Road ice is usually removed—or prevented from forming in the first place—using road salt. This is essentially large quantities of sodium chloride, our familiar table salt. When the ice melts, and the rains come, this salt is washed away into the river, making the water salty. Once the water supply was switched to the river, the concentration of chloride in the pipes increased dramatically. The result was twofold. First, the direct effect, was that GM shut shop. Second, the indirect effect, was far more critical. Chlorides corrode lead, and this causes the toxic metal to leach into the water.

Flint isn’t the only city to have lead pipes, or even moderated chloride concentrations in those leaden water pipes. However, unlike other cities, Flint neglected to pre-treat the water and add phosphates. Lead phosphate precipitates quickly and forms a protective layer on the inside of the pipes, thus preventing mishaps and crises like the one at Flint. Unfortunately, given that lead poisoning of water for human use has been a problem since Roman times, one wonders if another Flint is just around the corner.

History is such a good teacher, and humanity such an attentive and disciplined student, non¿ 6

Graphene microbots to the rescue?

Clearly, there is a need to clean up our water. Fast. Unfortunately, the task is neither easy nor cheap to undertake on a large scale. Filtration is, quite naturally, impossible, for the contaminants are molecular in size. Chemical means might work, but the by-products would then themselves have to be removed. Furthermore, since the cleaning chemicals would likely get used up, one would require a constant source of cleaning chemicals—an exercise that would lead, inevitably, to prohibitive costs.

Chemical means of removing lead3

In fact, chemical means would work, and are quite widespread. One direct method is to add chemicals so that lead salts would form, which are insoluble in water and quickly precipitate out. The problems : (a) how would one remove the precipitate? (b) how would one remove the chemical sludge that forms as a by-product? (c) this method does not work at low, but still dangerous, concentrations of lead.

A second method is to use synthetic organic resins as matrices that filter out lead ions from the water by exchanging ions with it. However, ion exchange is very sensitive to pH changes of the solution, and the method is useless if there is a high concentration of heavy metals or other organics, which tend to foul up the cleaning matrix.

Electrowinning, electrocoagulation and cementation are electrochemical methods of achieving the same goal. These are effective, but require a constant electric current and potential to be maintained. On large scales, this becomes prohibitively expensive.

An excellent self-explanatory diagram describing reverse osmosis.

Reverse osmosis (RO) and electrodialysis (ED) are both methods that use a forcing mechanism to push ions opposite to the natural osmotic direction. Osmosis is, of course, the process by which dissolved ions pass through a semipermeable membrane. Ions move from a region of high ionic concentration to one where the concentration is lower. Which means that distilled water connected to salty or contaminated water via a semipermeable membrane will slowly get saltier (or more contaminated) itself. In RO and ED, the reverse occurs. RO uses hydrostatic pressure as the forcing mechanism, while ED uses an electric field to encourage ions to move opposite to the osmotic pressure gradient. While these methods are useful in small scales such as individuals dwellings or groups of dwellings, they, too, quickly become too expensive on larger scales. Also, the higher cost of RO or ED systems takes them well out of reach of the poorer sections of the society, people who are at most risk from contaminated water.

And then there is adsorption.

No, not absorption, aDsorption.

Absorption is a phenomenon where one substance takes in another substance. It is a volume phenomenon, and takes place throughout the body of the “host”, if you will. Adsorption, on the other hand, is a surface phenomenon, and involves the interface between substances. The adsorbate sticks to (or adheres itself, to use more scientific language) the surface of the “host”. This sticky layer is often just a single molecule thick. The two processes are part of the more general sorption, whose opposite is desorption.

Gas–liquid absorption (a) and liquid–solid adsorption (b) mechanism. Blue spheres are solute molecules. (caption reproduced from source)

As the image shows, it is common for adsorbates to be solid molecules dissolved in a liquid that adhere to an adsorbing solid surface.

Common adsorbates

A photographer or an amateur astronomer always has packets or small sachets of silica gel lying around, especially in humid areas. Silica gel is an excellent drying agent, but it also moonlights as an adsorber of heavy hydrocarbons from natural gases. Zeolites are crystalline aluminasilicates that are highly porous and are used industrially as good adsorbers of CO2 and CO from natural gas. Activated carbon is carbon treated in a manner that it has lots and lots of pores, bit like zeolite, that increases its effective surface area hundredfold. For instance, depending on the method of manufacture, 1 gm of activated carbon can have surface areas4 of 1500 sqm to 3000 sqm. Activated carbon is the material of choice for most industrial and home purifier systems nowadays, and is believed to be most effective in the removal of lead. However, manufacturing activated carbon is expensive. Also, once it is used, the lead cannot be easily removed nor the carbon reactivated. In large scales, this creates the same problem as before : the cleaners have cleaned up, how do you now get rid of the cleaners?

Problem summary

Let’s sum up, and list out the problems faced by the cleaning efforts :

  1. Efficient cleanup

  2. Effective removal of the cleaning agents from the solution

  3. Reusability and regeneration of cleaning agent post-desorption

  4. Cost-effectiveness

The cleaning methods listed above fail in one or, as is more common, more of these criteria. It would seem there is no material or technique available that can at least tick three of these boxes.


Graphene microbots

Lead adsorption : Graphene oxide nanosheets

First things first : efficacy of cleanup. Lead in water comes in the form of divalent Pb(II) ions. What is needed is a material that would, in contact with contaminated water, reduce the Pb(II) concentration down to below 5% of the original. And its efficacy must not vary with either the concentration of Pb(II) or the pH7 of water. The solution, oddly, might come out of pencils.

Have you ever written using a pencil? I’d assume yes. If so, and if you have been reading this carefully, you’d be a bit confused. After all, we are attempting to remove lead. And, er, don’t pencils contain lead to begin with?

Well, if they did, we’d have generations of dead or deformed or diseased students and clerks and other…people who use pencils.

Pencil cores are made of a mixture of clay and graphite, an allotrope (another form) of carbon. Graphite is, essentially, loosely stacked atomic layers of carbon that peels away very easily. Graphene is any one such layer, and is one of the first truly 2D materials that humans have managed to come up with5. It had incredible electrical, optical and thermal properties that make it a must-material for the current futuristic age. And it is also very, very, very strong. Essentially, it is a supermaterial.

“It would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap.” — James Hone

Since graphene is 2D, it has a lot of surface area with respect to volume. Graphene oxide (GOx), also fabricated in nanosheet form, has a large number of oxygen functional groups on its abundant surface. These oxygen functional groups or moieties are capable of reacting with lead ions—actually, Pb(II) divalent lead ions—and forming surface complexes fast, adsorbing a lot of lead in quick time. Released in contaminated water in sufficient amounts, GOx would gobble up the local lead ions in a flash, leaving the water there quite pure. What is more, the initial Pb(II) concentration, whether large or small, would not matter. Also, if these GOx scrubbers are then treated with acid, the Pb(II) would be washed off, leaving them ready to be reused. This certainly solves Problem (3), but does so only partially, because it fails Problem (2) at the same time. Also, GOx nanosheets are only able to adsorb the Pb(II) ions that come in direct contact with it. It would be rather nice to have GOx nanosheets that would, perhaps by some chemical means, move about on their own in the solution, cleaning up the water as they swim, non?.

Chemical mobility : Platinum-catalysed oxygen microbubbles

Platinum (Pt) is an excellent catalyst, and has been employed as such since the early 19th century. In modern times, it is most widely used in catalytic converters in cars and automobiles, where it converts, catalytically, low concentrations of unburnt hydrocarbons in car exhaust into CO₂ and water vapour, thus decreasing environmental impact. It is also a catalyst in the following reaction

2 H2O2 + [Pt] → 2 H2O + O2 + [Pt]

If you add some platinum in a solution of hydrogen peroxide, you would immediately get a lot of bubbles from the released oxygen. If you create a spherical or cylindrical microcapsule of size about 10 µm or more, where one end is platinum and the other, say, silica, and if you put it in H2O2 solution of sufficient strength, you’d get a janus motor whose one end will keep producing microbubbles of oxygen. This will create a flowstream that will, through a phenomenon called diffusiophoresis, propel the Pt-SiO2 janus micromotor through the peroxide solution6. This means that, should one add H2O2 to contaminated water and somehow attach these self-propelled microswimmers to the GOx scrubbers, we would solve the previous problem, non?

A spherical janus micromotor. The hydrogen peroxide is catalysed by platinum, forming oxygen, microbubble streams of which provide propulsion.

Well, such a combination would certainly solve (1). But bear in mind that the micromotors are just that, motors. Or engines. They don’t have a steering wheel. Which means that, depending on the local H2O2 concentration gradients—which we cannot control effectively—they will zip about as they please. Removing and reusing them will not be possible on a sustainable basis.

That solves (1), but fails (2) and (4), and fails (3) partially, since it fails (2).

Magnetic steering : Nickel ferromagnetism

What we have now is a scrubber with a motor that does not have a steering wheel. Fortunately, Sámuel Sánchez and his co-workers at the Max Planck Institute for Intelligent Systems know how to proceed. Over to them now.

In a paper7 published in the journal Nano Letters, Sánchez and his team describe their cleaner microbots. These self-powered micromachines are hollow cylinders a tenth of the width of human hair8. They have four layers : an outer graphene oxide layer, two middle layers, and an inner platinum layer. The outer middle layer contains a platinum-nickel alloy, while the inner middle layer contains pure nickel. Once released in contaminated water that has been treated with H2O2, each layer performs crucial functions :

  1. Scrubber : The outer GOx layer adsorbs Pb(II).
  2. Motor : The inner hollow Pt layer propels the microbot via diffusiophoresis.
  3. Steering wheel : The middle Pt/Ni and Ni layers are ferromagnetic, and can be directed via magnetic fields.

This solves problems (1), (2), and (3). The GOx scrubber is efficient, especially if paired with the diffusiophoretic motor that enables it to race around the contaminated liquid. This solves (1). The magnetic steering wheel ensures that the microbots can be, post-scrubbing, led away from the now purified water to a safe place, thus solving (2). Treating the microbots with acid in this safe place removes the Pb(II), regenerating the adsorbing power of the GOx layer. This satisfies (2) and (3). Sounds good, non?

Structure of the GOx microbots, showing the four layers. Also the process of Pb(II) adsorption, propulsion via Pt-H2O2 catalysis, and eventual removal of Pb(II) by acid.

Well, yes. As a proof-of-concept, which is what all this really is. This fails (d) quite dramatically, because

  • The fabrication of graphene and graphene oxides on a large scale is quite expensive.
  • Treating a large mass of water with H2O2 is practical only if done so at source. Treating a whole contaminated lake with enough H2O2 ensure optimal microbot scrubber performance is, well, impractical at best.
  • The major problem is, of course, the steering. Magnets that can control microbot swarms over a large area are infeasible. This restricts the usage of the microbot scrubbers to pipes and restricted spaces, where magnets can be used locally.


The idea of a graphene oxide lead scrubber that moves on its own and can be controlled externally is quite, to use a technical term, cool. Sánchez et al have done a wonderful job by fabricating such a device. However, while it may have efficacy over small areas, it is impractical and just too expensive to implement over scales at which one finds contaminated water bodies that fall prey to industrial effluents and city sewage. The East Kolkata Wetlands is once such body of water. While it once had incredible natural filtration and cleanup capabilities, uncontrolled urbanisation, pollution and illegal landfills have reduced its efficacy dramatically. In order to tackle heavy metal pollution on such large levels, one needs to look at cheaper, simpler alternatives, such as microbial and plant derivatives (see 3 and references within), agricultural and household waste sorbents (see Table 3 of 8 and references within), and even the annoying9 water hyacinth plant9, through a process called phytoremediation10.

Which does not take any credit away from Sánchez et al, though. Who knows, it might well be possible to, one day, manufacture10 the GOx microbot scrubbers on a large scale and solve the H2O2-magnet problems too. Furthermore, since these scrubbers can be controlled very finely using sensitively tuned magnetic fields, they might be used to treat lead poisoning in human. The problem is, of course, H2O2, which is, as the joke goes…

Two men11 walk into a bar. The first, a chemist, tells the bartender, “I’ll have a glass of H2O please.” The second, not a chemist, listens to his friend and, impressed, tries to be smart. “I’ll have H2O too,” he says. The bartender, a moonlighting psychopath, gives each of them what they ordered. The non-chemist dies.

quite toxic to humans. 



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