PHYTO REMEDIATION Using nature to clean your soil

BROWNFIELDS TO GREENFIELDS A Field Guide to Phytoremediation.

PHYTOEXTRACTION:
Plants take up contaminants, mostly metals, metalloids and
radionuclides, with their roots and accumulate them in large quantities within
their stems and leaves. These plants are also called hyperaccumulators.

PHYTODEGRADATION:
Plants take up and break down contaminants through the
release of enzymes and metabolic processes such as photosynthetic
oxidation and reduction. In this process organic pollutants are degraded and
incorporated into the plant or broken down in the soil.

PHYTOVOLATILIZATION:
Some plants take up volatile contaminants and release
them into the atmosphere through transpiration. The contaminant is transformed or
degraded within the plant into a less toxic state and then released into the air.
Sometimes, the contaminant is released as is and then degraded by the sun.

RHIZODEGRADATION:
In some cases microbes in the soil break down contaminants
in to a less toxic state. In other cases these microbes can completely destroy
the contaminant. The root zones of certain plants create an environment that
assists this process. Therefore it is also called plant-assisted degradation.

RHIZOFILTRATION:
Some plant roots can filter contaminated water by adsorbing
the contaminants into their root and plant tissue. Similar to phytoextraction, the
plants themselves may become contaminated and have to be disposed as
special waste.

PHYTOSTABILIZATION:
Some plants can sequester or immobilize contaminants by
absorbing them into their roots and releasing a chemical that converts
the contaminant into a less toxic state. This method limits the movement
of contaminants through erosion, leaching, wind or soil dispersal. It is
often referred to as a „green cap“.

My only question is regarding PHYTOEXTRACTION. Geoff Lawton said that plants wont take up heavy metals unless conditions (soil/water) are less than 4.0 pH. Does this contradict that?

I think Geoff is making too broad a statement. I can think of one counterexample: Jimson weed has been observed to take up uranium and plutonium in the canyons around Los Alamos. And that soil is a lot higher than pH4.

A little searching on Google Scholar should turn up some insightful reading.

I think you're right.

I've been watching some of his Q&As from the new PDC and he's answered several questions about radioactive waste. His answers are a little over my head but he does talk about locking up pollutants in the carbon cycle. He said they were farming in Japan 5 years after Hiroshima and the top predator (wolves) are back at Chernobyl meaning that the system is substantially recovered if the top predator is back. So, nature (plants) can take care of much pollution but it'd be nice to know what part of the plant is taking up what pollutant to keep it out of the food stream as long as possible.

The pdf I linked does answer some of those specific questions.