Exercise 1 Heavy Metal Phytoextraction A plant accumulates a heavy metal to a bioconcentration factor of 10 (kg/kg dry weight based). There is a layer of 62.5 cm (density 1.6 kg/L = 1 ton per m 2 ) soil with 10 mg/kg of a heavy metal. Each year, the plant cover produces a net growth of 1 kg dry weight /m 2. How long will it take until the concentration in the contaminated soil layer has fallen to 1 mg/kg, if plant extraction is the only relevant removal process?

Exercise 1 Phytoextraction of Heavy Metals Bioconcentration factor BCF = C Plant / C Soil = 10 kg/kg Concentration in plant C Plant = BCF x C Soil Mass of soil M Soil = 1000 kg per m 2 Plant growth = 1 kg m -2 year -1 Initial mass of chemical in 1000 kg soil = 10 mg/kg x 1000 kg = mg Extraction first year = C Plant x Growth = BCF x C Soil x Growth = 10 x 10 mg/kg x 1 kg / year = 100 mg/year After one year, the concentration ins soil has fallen to 9900 mg/ton, yes. This is how many students try to solve the exercise. But what is the solution?

Solution of exercise 1 - the mass balance Change of heavy metal mass in soil per year dm/dt dm/dt = - BCF x Growth x C Soil with concentration C Soil = heavy metal mass m / soil mass M Soil dC Soil /dt = - (BCF x Growth / M Soil ) x C Soil = -k x C Soil The solution of this linear differential equation is always C Soil (t) = C Soil (0) x e -kt Now we need to find the time t when C Soil (t) = 1 mg/kg with k = - 10 x 1 / 1000 per year = 0.01 year -1.

Solution of exercise 1 - the result Equation C Soil (t) = C Soil (0) x e -kt = 10 mg/kg x e -0.01*t we seek for t when C Soil (t) = 1 mg/kg 1 mg/kg = 10 mg/kg x e -0.01*t 1 / 10 = e -0.01*t ln(1/10) = x t ln (10) = 0.01 x t t = ln(10)/0.01 years = 2.30 / 0.01 years = 230 years

Phytoextraction of heavy metals takes time! Don't argue - it's mathematics !

Phytoextraction Field Study 2012 PhD project Mette Algreen Nielsen ● Is extraction by trees a feasible way to remove heavy metals from polluted soils? ● Planting of poplars and willows in 1999 at the Valby sludge disposal ● Measurement of Cd, Cu, Ni, Zn in soil year 2001 and 2011 ● Measurement of Cd, Cu, Ni, Zn in the wood 2011 (10 years later) ● Mass balance: how much of heavy metals was extracted?

Site description Valby slam basin ● Disposal of sewage sludge over decades (> 3 m thick)

MetalTree C Soil 2001 (mg/kg) C Wood 2011 (mg/kg) Cdwillow Cuwillow Niwillow Znwillow Cdpoplar Cupoplar Nipoplar Znpoplar Phytoextraction Field Study 2012 Measured concentrations in soil before start and in wood after 10 years

Mass balance calculations Change of chemical mass in soil M wood willow = 1 kg/a = 10 kg/10a M wood poplar = 1.5 kg/a = 15 kg/10a Change of concentration in soil 0.7 m deep soil with  = 1.3 kg/L M Soil = 910 kg/m 2 Time to reach legal standard C(t) k = - (BCF x dM Wood / M Soil ) C(0) is conc. now C(t) is legal standard

Mass balance phytoextraction MetalTree C Soil 2001 mg/kg C Wood 2011 mg/kg m extracted (mg) dC Soil mg/kg % of C Soil Cdwillow Cuwillow Niwillow Znwillow Cdpoplar Cupoplar Nipoplar Znpoplar

Comparison 2001 to 2012 MetalTreeC Soil 2001 mg/kg C Soil 2011 mg/kg dC Soil calculated Cdwillow Cuwillow Niwillow Znwillow Cdpoplar Cupoplar Nipoplar Znpoplar The variation of measurements is far above any effect of phytoextraction

Time to reach legal standard (examples) MetalTreeC Soil 2011 mg/kg legal standard mg/kg BCFTime (years) Cdwillow Cuwillow Znwillow Nipoplar No way - phytoextraction is not an option, not with these trees. The Valby willow test site is now used as parking place (2013)

Summary phytoextraction of heavy metals ● Phytoextraction efficiency depends on the BCF and on the ratio biomass to soil mass: dC Soil /dt = - (BCF x M plant / M Soil ) x C Soil ● Lab experiments are over-optimistic, due to the high ratio plant mass to soil mass. ● Changes in the field trial at Valby were in the ‰ level. ● I am not aware of any successful phytoextraction of metals, but I know about 30 unsuccessful field studies.

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