Recycling Toxic Waste

Hemp has shown throughout history that it is an incredibly versatile and useful plant. Over the course of millennia humans have cultivated and bred the cannabis plant for both its fibers and for the phytochemical rich resins but there is another useful property of this plant that has only come to light relatively recently: Phytoremediation. What is phytoremediation? Well, in this context it means the accumulation and sequestration of heavy metal contamination in soil. Hemp just so happens to be extremely good at this which makes life difficult for those growing hemp for human consumption as heavy metals are a major human health hazard. On the other hand, perhaps this ability could be harnessed and put to good use? As it turns out, it can but the endeavor needs to be profitable in order to see wide adoption. Several researchers have looked at this puzzle over the years and now a team from Hunan Agricultural University have taken a dive into the pros and cons of multiple approaches.

Extracting the Metals

One of the most straightforward concepts is simply to extract and refine the metals that accumulated in the hemp biomass. After all, the areas surrounding mining activity are some of the most in need of phytoremediation and the desire to extract the metals is the genesis of the contamination. The authors referred to this as ‘plant metallurgy’ and could include several strategies including leaching the desired metals from composted biomass.

Extracting the energy

A common way to address heavy metal contamination in hemp biomass is to convert the biomass to energy. This is often done by simply combusting the dried, compacted biomass. There are major drawbacks to this method as many metals such as zinc will simply escape as vapor at high temperatures and go on to pollute the surrounding environment.

Extracting a value added product

The authors propose a novel approach to contaminated biomass. Instead of using it as a raw material for energy production or an ore for metal extraction they suggest the possibility of using it as a feedstock for metal loaded biochar. The process involves pyrolysis and the authors claim that it immobilizes the metals within the biochar, eliminating the concerns of spreading contamination by combustion and producing specific products depending on the specific metal contamination of the biomass. Metals on carbon are important catalysts, as an example that would be derived in this process. Another possible application is as an adsorbant.

The Bottom Line

The authors contend that metal contaminated biomass should be viewed as a valuable resource to be harvested rather than as a waste product. They think that the process they describe provides a good foundation in exploiting a valuable resource that can be harvested from the most polluted soils on earth.

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You can read the original paper here

1. Youzheng Chai, Anwei Chen, Ma Bai, Liang Peng, Jihai Shao, Jiayi Yuan, Cui Shang, Jiachao Zhang, Hongli Huang, Cheng Peng,
   Valorization of heavy metal contaminated biomass: Recycling and expanding to functional materials,
   Journal of Cleaner Production, Volume 366, 2022, 132771, ISSN 0959-6526,
   https://doi.org/10.1016/j.jclepro.2022.132771.