Transformation of contaminant metals in agricultural soil amended with digested sludge over time due to repeated plant growth

The agricultural use of sewage sludge is a beneficial way for recycling the abundant plant nutrients and organic matter contained within it. Unfortunately, sewage sludge also contains modest levels of contaminant metals. The sludge application can lead to the gradual accumulation of contaminant metals in the agricultural soil, potentially causing environmental and health risks. Therefore, current regulations require the monitoring of total soil metal concentrations following sludge application to prevent introducing excessive metals to the soil. However, these controls are being increasingly considered unduly conservative as they fail to account for the transformation of metal behaviour and fate in sludge-soil/sludge-soil-plant systems on the risk assessments. To improve the agricultural use of sewage sludge and better assess the associated risk, it is necessary to comprehensively understand the transformation behaviour of metals in the systems. To achieve this aim, four key issues are identified and addressed in this project through a systematic literature review and a series of pot trials.

(1) Based on the most recently published data, it is estimated that the annual global production of sewage sludge may rise from ~53 million tons dry solids currently to ~160 million tons if global wastewater were to be treated to a similar level as in the 27 European Union countries/UK. Industrial wastewater, domestic wastewater and urban runoff are the three main sources of contaminant metals in sewage sludge. Conventional treatment processes generally result in the partitioning of over 70% of metals from wastewater into primary and secondary sludges. Typically, the order of metal concentrations in sewage sludge is Zn > Cu > Cr ≈ Pb ≈ Ni > Cd. The proportion of these metals that are easily mobilised is highest for Zn and Ni, followed by Cd and Cu, then Pb and Cr. The sludge application to agricultural land will lead to elevated metal concentrations, and potentially to short-term changes to the dominant metal species in the soil. However, the speciation of sludge-associated metals will change over time due to interactions with plant roots and soil minerals and as organic matter is mineralised by rhizo-microbiome.

(2) Substantial proportions of Cu and Zn can become incorporated into the sludge matrix after the amending of sewage sludge with soluble metal salts. Surfaces of original digested sludge are close to the saturation limit when the added ratio of metals is within 2% (mmetals/mdry sludge solids). In the metal-amended sludges, Cu is predominantly distributed in the oxidizable fraction when the added ratio is within 2%, which is similar to the original digested sludge (indicated by BCR data). There is no dominant speciation for Zn in the original digested sludge, but it is mainly distributed in the exchangeable fraction after initially adding Zn to the original digested sludge at a ratio of 0.4%. The proportion of exchangeable Zn fraction can reach over 90% when the added ratio is 5% or 8%.

(3) Cu and Zn in the original digested sludge are primarily in metal sulphide phases formed during anaerobic digestion. When the amended sludge is mixed with the soil, about 40% of Cu(I)-S phases and all Zn(II)-S phases in the amended sludge are converted to other phases (mainly Cu(I)-O and outer sphere Zn(II)-O phases). Further transformations occur over time, and with plant growth. After 18 weeks of plant growth, about 60% of the Cu added as Cu(I)-S phases is converted to other phases, with an increase in organo-Cu(II) phases. As a result, Cu and Zn extractability in the sludge-amended soil decrease with time and plant growth. Over 18 weeks, the proportions of Cu and Zn in the exchangeable fraction decrease from 36% and 70%, respectively, in the freshly amended soil, to 28% and 59% without plant growth, and to 24% and 53% with plant growth.

(4) In the digested sludge-amended soil, spring barley absorbs Cu only from the immediate vicinity of the roots (<< 1mm), but Zn is taken up from further afield (> 1mm). In the rhizosphere Cu is predominately present as Cu(I) oxides or as Cu(II) absorbed/bonded to phosphate, whereas Zn is present as Zn(II) in inner-sphere complexes with metal oxide surfaces, Zn(II) sulphides or Zn(II) bonded to/incorporated into carbonates. Cu taken-up by spring barley roots is largely sequestered in the root epidermis and/or cortex predominately in the coordination environments similar to those seen in the rhizosphere. Only a small proportion of the Cu is translocated into the vascular tissue (where it is in the same two bonding environments). Zn taken-up by spring barley roots is present as Zn(II) sulphides, Zn(II) absorbed to/incorporated into carbonates, or Zn(II)-organic complexes. Zn is readily translocated from roots to shoots. Better understanding of these differences in the mobility and uptake of Cu and Zn in sludge-amended agricultural soil could be used to undertake element specific risk assessments.