Using Water Treatment Residual to immobilise lead for in-situ remediation of contaminated soil

Abstract

The potential for using waste materials to provide a sustainable means of remediating contaminated land is being explored. Water treatment residual (WTR) is a waste product generated by drinking-water treatment facilities worldwide and is commonly disposed of by landfill. However, its chemical composition highlights its potential as a sorbent of contaminants found in the environment. In this thesis, WTR was assessed in terms of its potential to immobilise lead (Pb), a common toxic contaminant found in the environment, in order to investigate its suitability as an in situ amendment in contaminated soil.
Aluminium-based and iron-based WTRs were sampled from water treatment works in north east England and characterised in terms of their bulk composition to gain an insight into the nature of this waste product and the variation that exists in their physicochemical properties. WTRs are predominantly composed of natural organic matter (NOM) and Fe or Al in oxyhydroxide form. Al-based WTRs contained 10-21% Al and 13-26% carbon; Fe-based WTRs contained 25-37% Fe and 13-27% carbon. Detailed characterisation of Fe-based Broken Scar WTR revealed that ferrihydrite was the dominant mineral form, with the material comprising ~70 wt% ferrihydrite finely intermixed with ~23 wt% NOM. WTR is considered as an organo-mineral composite with low surface area (4.1 m2/g) and microporous structure. The strong interactions between the NOM and mineral components are considered to protect both parties from degradation and create a relatively stable matrix which is able to function as an adsorbent over a range of environmental conditions.
The Fe-based WTR was highly effective at adsorbing Pb(II) from aqueous solution (max sorption capacity 139 mg/g). Pb sorption exhibited biphasic kinetics where initially fast sorption gave way to slow sorption, which was considered to be controlled by intraparticle diffusion into the microporous matrix of the organo-mineral composite. Pb sorption to WTR was highly pH dependent although it was capable of functioning as a sorbent over a wide pH range. Comparison of nine different WTRs revealed that all WTRs had a high Pb sorption capacity. In general, the sorption capacity of the Fe-WTRs was higher than that of the Al-WTRs.
Experiments were conducted to compare the sorption characteristics of WTR to that of its end-member components, ferrihydrite and humic acid (as a proxy for the NOM component). WTR
exhibited a lower sorption capacity in comparison to humic acid (200 mg/g) and ferrihydrite (170 mg/g). The sorption edge of WTR closely reflected that of ferrihydrite across the whole pH regime (pH 3-7), implying that WTR sorption behaviour is dominated by its Fe oxyhydroxide component. It is considered that the strong associations between the organic and mineral components in WTR that stabilise the composite are also responsible for reducing its reactivity. The organo-mineral composite nature of WTR may provide benefits in terms of long-term immobilisation of contaminants.
The ability of WTR to immobilise Pb and Arsenic (As) from real contaminated soil was investigated through plant growth trials. Soil amendments included wet WTR, dried WTR, compost and dried WTR-compost combinations. In general, all amendments improved plant growth and some treatments reduced metal uptake from the contaminated soil, indicating the amendments reduced contaminant bioavailability. Wet WTR treatments were more effective than dry WTR amendments at reducing plant uptake, and combination treatments yielded the most improved plant growth. The WTR had greater effect on reducing As uptake than Pb. Higher mobility of As may promote greater interaction between this contaminant and the amendment. Equally, wet WTR and compost may achieve greater reduction in contaminant uptake than dry WTR as a result of greater physical interaction, since humic and Fe components in compost and wet WTR may be more readily leached and redistributed within the soil matrix. This suggests that contact is the critical factor in this remediation strategy. Overall, these findings show that WTR has high potential to act as an immobiliser of Pb and other contaminants, for use in soil remediation. The key challenge for this in situ stabilisation method is likely to be achieving sufficient mixing for adequate interaction between the contaminants and the amendments.