2.4. Pot trial 1
Soil was amended with biosolids that were collected from a previous experiment that examined the effects of Ag + and AgNPs on a
simulated WWT plant [10]. Briefly, two Ag treatments – Ag + (as
AgNO 3 ) and AgNPs – were added to separate sequencing batch
reactors (SBRs) that contained activated sludge mixed liquor and
influent wastewater. A third SBR was used as a control (no Ag
added). Following SBR operation (aerobic stage), sludge was then
anaerobically digested. At the end of both the aerobic and anaerobic stages, sludge samples were collected and dried (40 ◦ C) for
use in PT1. The dominant Ag species in all biosolids treatments
were Ag-S bound species (>85% of Ag as Ag 2 S (bulk), Ag 2 S-NP or
Ag-glutathione [10]).
For PT1, the six biosolids treatments were added to soil at a realistic application rate (for Australia [33]) of 10 t/ha, corresponding
to 5.9 g biosolids/kg soil. The bioavailability of Ag in biosolidsamended soil was compared to soils that were directly exposed
to Ag + and ‘pristine’ AgNPs or Ag 2 S-NPs. For each biosolids treatment, two letters, ‘A’ or ‘N’, will be used as a prefix to indicate how
the
biosolids were produced (aerobically or anaerobically, respectively). Overall there were nine soil treatments (Table 2).
The total Ag
concentrations of soils in PT1 (Table 2) varied
between
treatments due to the different Ag concentrations in
biosolids (Table
2). This was unavoidable because the biosolids
application
rate was kept constant for each treatment. To enable
comparisons of results
between treatments at different Ag exposure concentrations (0.2–9.5 mg Ag/kg, see S1.1 for individual
concentrations), Ag + (as AgNO 3 ) was also added to 18 separate pots
to establish a dose vs. shoot-uptake curve for this soil. Based on the
measured shoot Ag concentrations from these treatments, a doseresponse curve was developed, enabling the results from all other
treatments (i.e. biosolids and NP suspensions) to be compared to
similar Ag + concentrations.