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?

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?

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.

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

Phytoextraction of heavy metals takes time!

0

2.5

5

7.5

10

0 100 200 300 400 500

Time (years)

CSo

il (m

g/kg

)

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)

Metal Tree CSoil 2001

(mg/kg)

CWood 2011

(mg/kg)Cd willow 7.7 3.1Cu willow 370 10.7Ni willow 89 0.48Zn willow 1800 106Cd poplar 11 0.62Cu poplar 717 1.7Ni poplar 170 0.1Zn poplar 3467 21.6

Phytoextraction Field Study 2012

Measured concentrations in soil before startand in wood after 10 years

Mass balance calculations

woodwood CMdt

dm

soilsoil MdmdC /

Change of chemical mass in soilMwood willow = 1 kg/a = 10 kg/10a Mwood poplar = 1.5 kg/a = 15 kg/10a

Change of concentration in soil0.7 m deep soil with = 1.3 kg/LMSoil = 910 kg/m2

k

tCC

t Soil

Soil)(

)0(ln

Time to reach legal standard C(t)k = - (BCF x dMWood / MSoil ) C(0) is conc. nowC(t) is legal standard

Mass balance phytoextraction

Metal Tree CSoil

2001 mg/kg

CWood

2011mg/kg

m extracted (mg)

dCSoil

mg/kg

% of CSoil

Cd willow 7.7 3.1 31 0.03 0.44

Cu willow 370 10.7 107 0.1 0.03

Ni willow 89 0.48 4.8 0.005 0.006

Zn willow 1800 106 1060 1.2 0.07

Cd poplar 11 0.621 9.3 0.01 0.09

Cu poplar 717 1.68 25.2 0.03 0.004

Ni poplar 170 0.0946 1.4 0.002 0.001

Zn poplar 3467 21.6 324 0.36 0.01

Comparison 2001 to 2012

Metal Tree CSoil 2001

mg/kg

CSoil 2011

mg/kg

dCSoil calculated

Cd willow 7.7 24 0.03Cu willow 370 588 0.1Ni willow 89 132 0.005Zn willow 1800 3270 1.2Cd poplar 11 11 0.01Cu poplar 717 651 0.03Ni poplar 170 153 0.002Zn poplar 3467 2964 0.4

The variation of measurements is far above any effect of phytoextraction

Time to reach legal standard (examples)

Metal Tree CSoil 2011

mg/kg

legal standard

mg/kg

BCF Time (years)

Cd willow 20 0.5 0.15 22 379Cu willow 588 500 0.015 9835Zn willow 3000 500 0.036 45 290Ni poplar 140 30 0.006 155 756

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:

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.

Any remaining questions to the topic

"phytoextraction of heavy metals" ?