Mechanisms and capacity of sun driven uranium removal in natural and nature-like constructed wetlands

Abstract

Prerequisites for energetically independent - sustainable- passive water treatment systems are net primary production and a relatively slow to intermediate and turbulence, homogenous, and continuous through flow. Nutrients can be recycled and -if necessary- subsequently artificially applied. However, the situation is different when only the limited sorption capacity of litter, formerly bio-processed or fossil organic carbon (e.g. peat) is used. High regenerative and productive floating and emerse species (like duckweed and common reeds) are only partially in contact with the water and are adapted evolutionary to stagnant or very slow flowing waters. However, they are able to up-concentrate the U in orders of magnitude in the roots through transpiration driven rhizofiltration, and even initiate bio-mineralisation processes. They produce very different degradable litter qualities depending on species and nutritional status. Hence, the U accumulated in duckweed is recycled fast due to microbial decay. Few other species of emerse helophytes produces decay resistant leaf litter. Macrophytic submerse vascular plant and algae are light limited under water and grow relatively slow. A few species remain in vegetative state during winter period. However, some of the species like macrophytic algae Charasp. posses a small ecological adaptation only to and very slow flowing waters, but more have a high U sorption capacity, like microalgae. However the latter grow very fast and are associated in biofilm (periphyton on above named macrophytes and litter) in flowing water, which have a U removal capacity of up-to some hundred mgm^-2d^-1. Furthermore, most submerse. synthetic active organisms influence changes in milieu conditions (e.g. decrease of PO4 ^3- concentration, exudation of bioligands, CaCO3 precipitation by CO2 uptake and pH change) leading to U complexation and co-precipitation. Under suitable sedimentation conditions in slow flowing water, U can be removed by bio-colloids and particulate organic matter (CPOM, FPOM) from detached biofilms and decayed litter. Furthermore, microbial redox-process concentrate up U in sediment but depending on conditions (organic matter quality and quantity in relation to eacceptors hierarchy and supply, respectively). Based on own measured data and a meta-analysis for slight acid to alkaline and hard, it is estimated that the elimination capacity of the whole system can reach as high as some hundred g U m^-2 annum^{- 1} in temperate climate conditions. Hence, U elimination in sun driven constructed wetlands is possible, however, under a limited set of conditions and needs a patched community approach (ecological engineering in strict sense).