The plant that extracts arsenic is a fern. It ought to be possible to plant this kind of fern in the arsenical soil and harvest the grown plant. The plant material could be trucked to a waste site. Any arsenic in the plant is no longer in the soil, and a few years of this treatment should decontaminate the soil. Unfortunately, ferns are not able to grow in most soil conditions.
The life cycle of a fern is quite interesting. We are used to seeing the green plant with fronds, which biologists call a sporophyte, and it looks rather like many other plants. But its method of reproduction is quite different from more familiar plants.
Like ordiinary plants (and animals), each cell of a fern has two of each kind of chromosome. Biologists call it diploid.
The underside of its leaves can bear spores, but spores are not like seeds. Each spore is more like a grain of pollen than like a seed because it has only one of each kind of chromosome instead of two. Biologists call that haploid. Pollen, ovules, sperm cells and egg cells are haploid cells. Pollen and ovules come together to form a seed which has two of each chromosome, just as sperm and egg come together to form an embryo whose cells have two of each chromosome. But a successful spore grows into a complete multicellular plant called a gametophore , and each cell of the gametophore is also haploid. We usually don't notice it because it is usually very small. It doesn't look anything like a fern.
The gametophore lives its brief life and when conditions are right, it produces reproductive cells, also haploid, called gametes. Some gametes can swim like sperms and others, eggs, stay put. Eventually a sperm gamete can meet an egg gamete and they will fuse. The result of the fusion is now a diploid cell -- it has a pair of each kind of chromosome, one from each gamete. It is the fused gametes that grow to become a fern (diploid).
Imagine what humanity would be like if every sperm cell could grow into a complex individual and have numerous children of its own, and if those children could mate, like a sperm and an egg, to become a human being. Our children would not resemble us at all, but our grandchildren would. It seems like it would be a good plot for a science fiction story.
The gametes must swim so at this point of the fern's life cycle, it needs a watery environment. Ferns cannot grow in dryish soils. So they wouldn't be suitable to decontaminate dry soils.