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.

 

Introduction

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.

Non?

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.

Conclusion

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. 

Oui¿

References

1.
Cullen, W. Is Arsenic an Aphrodisiac? (Royal Society of Chemistry, 2008).
2.
Zhao, H.-L., Zhu, X. & Sui, Y. THE SHORT-LIVED CHINESE EMPERORS. Journal of the American Geriatrics Society 54, 1295–1296 (2006). [Source]
3.
Ahluwalia, S. S. & Goyal, D. Microbial and plant derived biomass for removal of heavy metals from wastewater. B 98, 2243–2257 (2007). [Source]
4.
Dillon, E. C., Jr, Wilton, J. H., Barlow, J. C. & Watson, W. A. Large surface area activated charcoal and the inhibition of aspirin absorption. A 18, 547–552 (1989). [Source]
5.
Geim, A. K. & Novoselov, K. S. The rise of graphene. N 6, 183–191 (2007). [Source]
6.
Zhang, J., Zheng, X., Cui, H. & Silber-Li, Z. The Self-Propulsion of the Spherical Pt–SiO2 Janus Micro-Motor. M 8, 123 (2017). [Source]
7.
Vilela, D., Parmar, J., Zeng, Y., Zhao, Y. & Sánchez, S. Graphene-Based Microbots for Toxic Heavy Metal Removal and Recovery from Water. N 16, 2860–2866 (2016). [Source]
8.
De Gisi, S., Lofrano, G., Grassi, M. & Notarnicola, M. Characteristics and adsorption capacities of low-cost sorbents for wastewater treatment: A review. S 9, 10–40 (2016). [Source]
9.
Ghosh, M. & Singh, S. P. A review on phytoremediation of heavy metals and utilization of its by products. Asian J Energy Environ 6, 18 (2005). [PDF]
10.
Salt, D. E. et al. Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants. N 13, 468–474 (1995). [Source]
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Footnotes

  1. To paper-and-pen campaigners I would say this, “Be practical.”
  2. I did say “one of the…”.
  3. http://bigbangtheory.wikia.com/wiki/The_Big_Bran_Hypothesis
  4. While arsenic isn’t technically a metal, it is often included among heavy metals. Goodness knows why.
  5. One of the five great lakes of the USA, the most central one.
  6. Means no in French.
  7. Measure of acidity/alkalinity. 7 is neutral, less is acidic, more is alkaline.
  8. Outer diameters are 4.6 µm on average, while a typical human hair is 25.4 µm in diameter.
  9. To some. To others, it is beauty.
  10. As opposed to fabricate, which is more of a small-scale process
  11. Women are never this stupid.

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