The effects of sewage sludge and sewage sludge biochar on PAHs and potentially toxic element bioaccumulation in Cucumis sativa L.

Highlights

• Biochar reduced bioaccumulation of PAHs and PTEs.

• Sludge reduced PAHs bioaccumulation but increased PTE bioaccumulation.

• Bioavailability reductions were significantly greater for biochar vs. sludge.

• Bichar had greater potential over SS in increasing cucumber fruit biomass.

• Biochar could be a promising alternative to SS as a soil amendment.

Abstract

The presence of contaminants such as polycyclic aromatic hydrocarbons (PAHs) and potentially toxic elements (PTEs), including As, Cd, Cu, Pb and Zn, restricts the application of sewage sludge (SS) to agricultural land. This research established that the conversion of SS to SS biochar (SSBC) significantly (p ⩽ 0.01) decreased PAH and available PTE concentrations. Once added to soil both SS and SSBC significantly (p ⩽ 0.05) decrease PAH availability. Bioaccumulation of PAHs into Cucumis sativa L. was reduced by both SSBC (44–57%) and (to a lesser extent 20–36%) by SS. Following addition to soil SSBC significantly (p ⩽ 0.05) reduced available PTEs (except Cd), while SS significantly (p ⩽ 0.05) increased PTE availability. As a consequence SSBC significantly (p ⩽ 0.05) reduced PTE bioaccumulation (except Cd and Zn), while SS increased PTE bioaccumulation. These results suggest SSBC to be a candidate for soil amendment that offers advantages over SS in terms of PAH/PTE bioaccumulation mitigation.

Introduction

Sewage sludge (SS) is produced in massive quantities as a byproduct in wastewater treatment plants (WTPs). In China, WTPs produced approximately 30 Mt of SS per year (Yu, 2011). This quantity is expected to increase in keeping with China’s rapid urbanization and industrial development. SS contains high concentrations of micro- and macro-nutrients and the agronomic benefits of SS application to land are well recognized (USEPA, 1995, Roca-Pérez et al., 2009). However, the presence of toxic contaminants such as polycyclic aromatic hydrocarbons (PAHs), potentially toxic elements (PTEs) such as As, Cd, Cu, Pb and Zn and pathogens restricts the application of SS to farmland (USEPA, 1995, SEPA, 1995, CEC, 2000).

Concerns regarding PAHs and PTEs in the environment are well founded. PAHs form adducts with DNA and have, as a consequence, been prioritized by the US EPA and EU on account of their carcinogenetic, mutagenic and teratogenic properties (White and Claxton, 2004). Similarly, where PTE exposure exceeds critical thresholds, their toxicity (giving rise to: cancer, neurological degeneration, muscular dystrophy, and multiple sclerosis) is manifested (Järup, 2003). Furthermore, it is well documented that elevated levels of PTEs can adversely affect abundance and diversity of soil organisms (Creamer et al., 2008, Uchimiya et al., 2011a). Indeed, elevated PTE concentrations have been suggested to have lasting effects on soil ecosystem functioning (Creamer et al., 2008). The levels of PAHs and PTEs in a range of biochars have recently been reported (Uchimiya et al., 2011a, Freddo et al., 2012, Hale et al., 2012). Encouragingly, levels of both PAHs and PTEs in biochars (but not sewage sludge biochar (SSBC) specifically) have been found to be comparable with those found in background soils and below levels used in the regulation of composts (Freddo et al., 2012).

Through the conversion of SS to SSBC it may be possible to decrease PAH concentrations and PTE availability. Thus, the conversion of SS into SSBC represents an interesting opportunity to potentially mitigate PAH/PTE issues relating to SS (while at the same time retaining the soil improvement properties associated with SS) and in so doing to produce a safer soil amendment for application to agricultural land. In addition to the possibility of mitigating toxicity associated with SS through its conversion to biochar wider environmental benefits could be realized (particularly given the ready availability of SS); most significant would be the opportunity to divert SS waste to use in heat and power generation.

Specifically, biochar, on account of its sorptive capacity, could be an effective soil amendment to reduce the bioavailability of contaminants such as PAHs and PTEs (Cao et al., 2011, Khan et al., 2013, Sneath et al., 2013). Regarding PAHs biochar application to contaminated soil and SS has been shown to cause significant decreases in freely dissolved PAH concentrations (Oleszczuk et al., 2012, Khan et al., 2013). The mass transfer of PAHs to biochar sorptive sites has been proposed as the mechanism through which this is achieved (Uchimiya et al., 2011b). With respect to PTEs the oxygen-functional-groups of biochar have been reported to form complexes with PTEs which reduce their mobility and their bioaccumulation in plants grown in biochar amended soil (Uchimiya et al., 2011b, Jiang and Xu, 2013).

While bioaccumulation of PAH/PTE into plant tissue (rice shoot/root and lettuce, respectively) following biochar application to soil has recently been reported (Zheng et al., 2012, Khan et al., 2013). Furthermore, the direct comparison of how SS and SSBC perform with respect to each other has not. Cucumber (Cucumis sativa L.) was chosen as a representative plant belongs to cucurbitaceae family. In China cucumber is an economically important plant (with 4.74 Mt being produced each year; this making China the largest producer in the world) (FAO, 2011). Cucumber grown in the field can become contaminated with organic compounds and PTEs with these elevated concentrations giving rise to health problems (Mansour et al., 2009).

It was the aim of this research to assess the extent to which conversion of SS to SSBC decreased PAH concentrations and the availability of PTEs. Thereafter, it was the aim of this research to establish the relative effectiveness of SS and SSBC, once added to contaminated soil, to: (i) sustain plant growth, (ii) alter the partition of PAHs and PTEs, and (iii) influence the bioaccumulation of PAHs and PTEs into a commonly cultivated salad crop (C. sativa L.).