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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 Rich on Tuesday February 01 2022, @01:49PM (18 children)
So you move the trace amounts of heavy metals into large amounts of organically grown biochar. After that it's no longer biochar, but toxic-char, and a lot of that. Where do you get rid of that?
Maybe they could hydrate the waste with bio-hydrogen, siphon off the resulting alkanes and then process the smaller amount of residue for whatever is left over?
(Score: 0, Disagree) by Anonymous Coward on Tuesday February 01 2022, @02:14PM (8 children)
They used to promote it for carbon capture and to increase soil fertility. It didn't do either. Now they're trying to push it for this. I've got no idea why some people are so hung up on effing charcoal.
(Score: 2) by ElizabethGreene on Tuesday February 01 2022, @05:11PM (7 children)
Do you have any additional data on Biochar failing to increase soil fertility? I haven't followed up on that research in a while and my recollection is that the results were promising.
(Score: 1, Interesting) by Anonymous Coward on Tuesday February 01 2022, @08:10PM (6 children)
I don't feel like Googling it right now.
After the hype, experiments were carried out by universities in the US for fertility changes with biochar added. Sometimes the biochar had no effect on fertility, and sometimes it decreased it. I don't think there were any significant cases of biochar making the soil more fertile. What you have to bear in mind is that biochar is quite porous and tended to lock away nutrients. bio char proponents are trying to mimic the fertile soils found in some parts of the Amazon jungle known as "terra preta do indio", or Indian black earth. These deposits are the result of basically garbage pits where a lot of organic matter was burned and tossed.
As for carbon capture: the bio char is not that much in terms of mass for storing carbon and the carbon does not stay locked in place either.
(Score: 0) by Anonymous Coward on Tuesday February 01 2022, @08:13PM
I believe the nitrogen was temporarily reduced after biochar was applied because the biochar encouraged its own decomposition. Could be wrong about the exact reason, but frequently, nitrogen was lowered in the soil.
(Score: 2) by DeathMonkey on Tuesday February 01 2022, @10:04PM (4 children)
Maybe you should've tried that googling because the vast majority of the references I'm seeing indicate that it does work.
For example:
Biochar addition persistently increased soil fertility and yields in maize-soybean rotations over 10 years in sub-humid regions of Kenya [sciencedirect.com]
(Score: 0) by Anonymous Coward on Wednesday February 02 2022, @12:12AM (3 children)
Experiments in the US which already has well-managed soil as well as a temperate climate have not shown a benefit.
Maybe there is some benefit in specific climates where the soil is not being well managed. In the US, it is an old, common practice to apply "green manure" which is a crop of clover which is grown and then tilled back into the soil. This a accomplishes a few things: it adds nitrogen to the soil, adds carbon, and adds structure to keep the soil from compacting. Maybe the biochar is doing something similar in a tropical climate, but where the nitrogen would come from is only a guess.
Biochar is not GENERALLY useful to increase crop yield. The experiments were done by ag depts in midwestern universities.
As for carbon sequestration, that is just a joke; there isn't much biochar mass you can add to a plot of soil. Growing trees capture far more carbon per acre.
(Score: 0) by Anonymous Coward on Wednesday February 02 2022, @07:11AM (2 children)
>As for carbon sequestration, that is just a joke; there isn't much biochar mass you can add to a plot of soil. Growing trees capture far more carbon per acre.
First you grow the trees then you turn then into charcoal and plant new trees. There's your sequestration.
(Score: 0) by Anonymous Coward on Wednesday February 02 2022, @12:46PM
An acre can store much more carbon in tree form because the trees are vertical. Scattered into chips and placed just under the soil as biochar is a far less dense packaging. Plus carbon stays locked in trees for many, many decades. In the soil, it can erode away. Plus you'd have to plow an entire field to bury the biochar, whereas with trees you need only dig a small hole per tree, if that.
(Score: 0) by Anonymous Coward on Friday February 04 2022, @07:42PM
(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.
(Score: 2, Interesting) by istartedi on Tuesday February 01 2022, @05:08PM (4 children)
Phytoremediation isn't new so I'm sure this has been thought of. I think the concentration of metals in the plants is considerably less than the soil they leach from. The upside is that the product isn't too toxic. The downside is that remediation takes several cycles. In the unlikely event it's so toxic that it's dangerous, you landfill the stuff. In most cases "the solution to pollution is dilution". The heavy metals in toxic sites would eventually diffuse in to the environment and not have a significant impact on the average concentration. They're just accelerating that process.
Appended to the end of comments you post. Max: 120 chars.
(Score: 3, Informative) by ElizabethGreene on Tuesday February 01 2022, @05:19PM (2 children)
This isn't phytoremediation. The core process they describe is cleaning water with carbon like you do in a fishtank filter. That they are using biochar from banana waste biomass is noise around the edge of the core process.
The interesting comparison I didn't see in the paper is "How does this compare to Activated Carbon"? If it costs 10% of what AC costs and works 90% as well then that would be very interesting.
(Score: 3, Informative) by DeathMonkey on Tuesday February 01 2022, @05:22PM (1 child)
They don't quantify costs but they believe it will be cheaper since it's sourced from a waste stream. I think they wanted to find out if it actually works first.
FTA
(Score: 3, Informative) by hubie on Tuesday February 01 2022, @05:49PM
That is the part that I found interesting about this paper. They're starting from an existing waste stream and also state that banana stems and leaves contain high lignin and low cellulose, which makes them more preferred candidates over other biomaterials for making biochar material that is highly porous with high fixed carbon content.
(Score: 2) by DeathMonkey on Tuesday February 01 2022, @05:19PM
This isn't phytoremediation.
They're just making charcoal filters out of banana industry waste, basically.
(Score: 2) by DeathMonkey on Tuesday February 01 2022, @05:17PM
Because the lead bonds with the biochar it is no longer leachable. So you can bury it safely* and it won't get into your groundwater.
*Relatively speaking, you want to make sure the char itself doesn't escape because it does carry metals. But, it's a lot easier to keep solids in place compared to liquids.
(Score: 1, Troll) by crafoo on Tuesday February 01 2022, @06:05PM
You just need to find somebody not-you to sell the bananas to. What's the problem?? ship them off for free. someone will take them. maybe China needs to feed their prisoners on the cheap.