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from the Six-foot-seven-foot-eight-foot-bunch! dept.
Heavy metals contaminate ground and surface waters from a variety of sources such as industrial effluent or fertilizers or pesticide applications. Cadmium and lead are the most common and toxic metals found in aqueous environments. They are persistent, they migrate, they accumulate in biological tissues, and they are carcinogenic. Removing these metals effectively and cheaply has been a big environmental challenge. There are a number of approaches to remove them including reverse osmosis, ion-exchange, chemical precipitation, coagulation, electrochemical treatment, and physical adsorption. Of these, adsorption is seen as very promising due to it being cost-effective, widely available, and easy to implement. There are a wide variety of adsorbent materials from the mundane (activated carbon, diatomaceous earth, polymers, etc.) to the exotic (carbon nanotubes and graphene oxide), but biochar has shown to be very efficient and cost-effective.
Biochar is generated from incomplete combustion of organic material at low temperatures under oxygen-starved conditions. It can be made using any organic material, such as forest and crop residues, algae, etc., and it results in a material with unique physiochemical properties such as producing a very porous material with abundant functional groups that bind to the metals. A group of researchers investigated the effectiveness of biochar made from banana waste, particularly the stem and leaves. They chose bananas because it is the fourth-most grown crop in the world. After a harvest, the stems and leaves are discarded in the field. Since the bananas only make up about 12% of the plant mass, this means a significant amount of biowaste is generated. They found that they could recycle the banana waste residues effectively for preparing adsorbents for treatment of heavy metals in contaminated water, and they hope that this would promote agricultural waste recycling as well as providing material for treating contaminated water.
Absorption at Wikipedia.
Journal Reference:
Xiyang Liu, Gaoxiang Li, Chengyu Chen, et al. Banana stem and leaf biochar as an effective adsorbent for cadmium and lead in aqueous solution [open], Scientific Reports (DOI: 10.1038/s41598-022-05652-7)
(Score: 3, Interesting) by Michael on Tuesday February 01 2022, @02:46PM (1 child)
Not sure what you're basing your characterisation of the amounts involved from, or how the biochar is sourced, but it doesn't reflect the experiments they actually did. It's all very well having a gut feeling about particular subjects, but it's also well to check if the conclusions you jump to make sense or correspond to the empirical evidence.
Skimming the paper, it looks like a gram would adsorb the lead from a couple of thousand grams of water contaminated with a thousand times the epa drinking limit of lead. So unless you're defining organically grown as "any", lots as "0.05% per unit weight" and trace amounts as "your kids are now retarded", I don't see how that can be supported by the evidence.
(Score: 3, Informative) by Rich on Tuesday February 01 2022, @05:18PM
They give 300mg/g adsorption limit for lead (much less for Cd), with the kinetics falling of past 150mg/g saturation, your (arbitrary, and pretty high) 1000x level (of the EPA 15ug) is 15mg/l. So at that level our g of bananacoal can de-tox 10000g = ~10l of water. But unless you're selling Evian, water is dealt with in cubic metres. And at that rate, 1m^3 of water will produce a pound of toxic waste that has to be dealt with further. For reference, the German Tesla factory will use over 1M m^3/a of water, so at such an industrial scale, we would end up with a million pounds (or 500 tons) of waste that have to be dealt with. I concede that it's probably not economical (at least for lead filtering) to run a large scale hydration plant and it might be more efficient to use the remaining carbon as a heat source to smelt out the 75 tons of lead. But with 5000 tons (as for Cd), I'd like to see the numbers run.