Exercise 1 Heavy Metal Phytoextraction

1. 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 m2) soil with 10 mg/kg of a heavy metal. Each year, the plant cover produces a net growth of 1 kg dry weight /m2. 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?

2. Exercise 1 Phytoextraction of Heavy Metals Bioconcentration factor BCF = CPlant / CSoil = 10 kg/kg Concentration in plant CPlant = BCF x CSoil Mass of soil MSoil = 1000 kg per m2 Plant growth = 1 kg m-2 year-1 Initial mass of chemical in 1000 kg soil = 10 mg/kg x 1000 kg = 10 000 mg  Extraction first year = CPlant x Growth = BCF x CSoil 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?

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

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

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

6. 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?

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

8. Phytoextraction Field Study 2012 Measured concentrations in soil before start and in wood after 10 years

9. Mass balance calculations Change of chemical mass in soil Mwood willow = 1 kg/a = 10 kg/10a Mwood poplar = 1.5 kg/a = 15 kg/10a Change of concentration in soil 0.7 m deep soil with r = 1.3 kg/L MSoil = 910 kg/m2 Time to reach legal standard C(t) k = - (BCF x dMWood / MSoil ) C(0) is conc. now C(t) is legal standard

10. Mass balance phytoextraction

11. Comparison 2001 to 2012 The variation of measurements is far above any effect of phytoextraction

12. Time to reach legal standard (examples) No way - phytoextraction is not an option, not with these trees. The Valby willow test site is now used as parking place (2013)

13. Summary phytoextraction of heavy metals ● Phytoextraction efficiency depends on the BCF and on the ratio biomass to soil mass: dCSoil/dt = - (BCF x Mplant/ MSoil) x CSoil ● 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.

14. Any remaining questions to the topic "phytoextraction of heavy metals" ?