The seeds from a common tree species native to Asian countries could provide a novel means of filtering the microplastics increasingly present in human drinking water supplies.
The seeds of the moringa, or white acacia, can be used to make a saline extract, which solidifies water containing microplastics, thereby trapping it.
The “miracle tree” is already extensively used by humans for food, through the consumption of its leaves and seeds, which are both nutritious, but for several years researchers have been studying the potential for the seeds to be used in water treatment.
Co-author of the study, Gabrielle Batista, from São Paulo State University, said: “We showed that the saline extract from the seeds performs similarly to aluminum sulfate, which is used in treatment plants to coagulate water containing microplastics. In more alkaline waters, it performed even better than the chemical product.”
The study focused on water treatment via in-line filtration. In this process, the water is coagulated, which destabilises the particles, and then it passes through a sand filter.
Coagulation is essential because pollutants, such as microplastics, have a negative electrical charge on their surface and repel each other and the sand in water treatment filters.
Coagulants, such as moringa salt extract – which can be made at home – and aluminum sulfate, neutralise this charge. This causes the pollutants to clump together so they can be filtered out.
Professor Adriano Gonçalves dos Reis, who co-ordinated the research, said: “There’s increasing regulatory scrutiny and health concerns regarding the use of aluminum- and iron-based coagulants, as they aren’t biodegradable, leave residual toxicity, and pose a risk of disease. For that reason, the search for sustainable alternatives has intensified.”
To test how effective the moringa seed water treatment is, the researchers used tap water which they purposefully contaminated with polyvinyl chloride (PVC).
PVC and the microplastics which come from it are one of the most dangerous plastics for human health due to its documented mutagenic and carcinogenic potential. Unfortunately, PVC is already highly prevalent on the surfaces of water bodies and in water treated by traditional processes.
The team also artificially aged the PVC using ultraviolet radiation to mimic natural processes such as degradation from sunlight and from the water, and reproduce the properties of naturally aged microplastics.
The contaminated water was then coagulated using the moringa seeds and filtered in a Jar Test – a device that replicates water treatment processes on a small scale.
The results were then compared to those of the same tests performed on water treated with aluminum sulfate.
Scanning electron microscopy (SEM) was used to count the microplastic particles before and after treatment, and the team reported “no significant differences” in particle removal were found.
The group is now testing moringa seed extract using river water. They said that so far, their experiments show the moringa product “has proven quite effective in treating natural water” too.
The research is published in the journal ACS Omega.