Content and morphology of lead remediated by activated carbon and biochar: A spectral induced polarization study

Highlights

• Spectral induced polarization (SIP) used to monitor Pb remediation by AC & biochar.

• SIP revealed the evolution of size and content of the retained Pb.

• SIP-derived characteristic grain/pore size agrees well with SEM and MIP results.

• Precipitation and physical adsorption are the major Pb remediation mechanisms by AC.

• Surface complexation is the major Pb remediation mechanism by biochar.

Abstract

Soil and groundwater contamination with lead (Pb) poses serious challenges for the environment. Activated carbon (AC) and biochar have huge potential application in the in-situ remediation processes through permeable reactive barriers (PRB). Spectral induced polarization (SIP) technique recently showed promises in nondestructively monitoring the spatio-temporal characteristics of physical, chemical and biological processes in porous media. In this study SIP technique was used for monitoring Pb remediation by AC and biochar in column scale. The calculated characteristic grain/pore size evolutions from SIP signals on AC, agreed well with the size of precipitates measured by SEM and mercury intrusion porosimetry (MIP) methods. The content increment process of the retained Pb on AC was also recorded via the magnitude increment of the imaginary conductivity. The mechanisms of Pb–AC and Pb-biochar interactions were investigated using SEM-EDS, TEM, FTIR, XRD, and XPS measurements. It showed that AC immobilizes through physical adsorption and precipitation, whereas complexation with functional groups is the remediation mechanism for biochar. Furthermore, the observed SIP responses of both AC and biochar are two orders of magnitude higher than those of typical natural soils or silica materials. This distinct difference is an additional advantage for the field application of SIP technique in PRB scenarios.

Introduction

Heavy metal pollution at mining operation sites, municipal landfill waste (MSW) sites, and industrial factories raises serious concerns for policy makers, scholars, and practitioners owing to the high toxicity and non-degradability characteristics of heavy metals. The remediation on heavy metals have been developed covering chemical (e.g., chelate) (Wang et al., 2019), physical (e.g., leaching) (Löser et al., 2007) to microbiological (e.g., biotransformation) (Zhang et al., 2013) and even electrical techniques (Buchireddy et al., 2009). Lead (Pb) is a toxic heavy metal that can cause severe physiological or neurological damage to the human body even with only a trace amount present in water (Han et al., 2018). The US EPA has a goal of a Pb level in drinking water of zero, which mandates highly efficient remediation techniques for Pb contamination (Epa, 1991). Thus, barriers containing existing contaminated sources, such as waste rock piles from metal mining operations or MSW sites, are appealing to practitioners.

In situ treatment approaches, such as permeable reactive barrier (PRB), immobilization and stabilization techniques, are being developed for either removing heavy metals from the aqueous phase or temporarily stabilizing them in contaminated sites (Liu et al., 2015, Statham et al., 2016, Ye et al., 2019, Yamada and Katoh, 2020, Zhang et al., 2020). Recently, carbonaceous sorbents have demonstrated promising applications in PRB reactive and immobilization materials during remediation of contaminated sites (Ye et al., 2019), among which activated carbon (AC), biochar and carbon nanotubes (CNTs) have been extensively studied (Daneshvar Tarigh and Shemirani, 2013, Rizwan et al., 2016, Zabihi et al., 2009, El-Shafey, 2010, Mahmoud et al., 2012, Inyang et al., 2016, Yang et al., 2019, Baccar et al., 2009, Wang et al., 2018). The remediation ability and efficiency of carbonaceous materials on heavy metals mainly depend on the material surface and structure properties. CNTs, an emerging carbonaceous member, display various efficiencies on Pb adsorption. For instance, the pre-oxidized CNTs could have adsorption capacity on Pb as high as 97.1 mg/g, since the oxidizing process has increased the surface functional groups and nanostructures (Daneshvar Tarigh and Shemirani, 2013). Comparatively, AC (Shi et al., 2019, Goel et al., 2005, Issabayeva et al., 2006) and biochar (Tan et al., 2015, Shen et al., 2019, Inyang et al., 2012) are mostly reported of higher removal efficiencies with less modification and cost. Both of them are solid products from pyrolysis of waste biomass residues in oxygen-limited environments from agricultural and forestry production. Compared to biochar, AC is produced at relatively higher temperatures, followed by an activation procedure, which yields a higher surface area, pH, and fewer surface functional groups (Huggins et al., 2016, Rostamian et al., 2015). Unlike AC, biochar usually contains a non-carbonized fraction and the extent of O-containing carboxyl, hydroxyl, and phenolic surface functional groups which could effectively bind water contaminants (Uchimiya et al., 2011). The different production processes between AC and biochar may induce their remediation discrepancy (Ahmad et al., 2014). However, few studies were focused on the Pb remediation mechanisms by AC and biochar, and their differences. Furthermore, although the adsorption, kinetics and equilibrium properties of AC and biochar were acquired through lab-scale experiments (Kumarasinghe et al., 2018, Mohan et al., 2011, Paranavithana et al., 2016, Kumarasinghe et al., 2018, Di Natale et al., 2008), the evaluating and monitoring of their long-term performance and life expectancy as PRB filling materials are challenging.

The existing monitoring method for contaminants characterization and remediation evaluation has significant limitations: for instance, traditional water sampling and testing from the often scarcely-distributed observation wells downstream can only show current contamination status without the ability to predict future breakthrough time, and suffer from limited spatiotemporal heterogeneity information, high testing costs, and long turnaround time. High-density electrical resistivity tomography technique can measure the spatial heterogeneity of the DC electrical conductivity, which reveals only the bulk electrical signatures of soil, contaminants (precipitates) and pore fluid without further distinction (Johansson et al., 2017, Power et al., 2018). Spectral induced polarization (SIP) technique is a well-established geophysical method for characterizing the subsurface structure and exploring for metallic deposits. Recent years, SIP began to be used in hydrogeological and environmental investigations, particularly for fluid content, fluid chemistry and contaminant transport. It has the ability to non-destructively evaluate the fate and transport of pollutants spatiotemporally, for instance, calcite precipitation and deposition (Wu et al., 2010, Zhang et al., 2012), Pb and zinc adsorption (Vaudelet et al., 2011), and microbial-induced sulfide precipitation in porous media (Ntarlagiannis et al., 2005, Mellage et al., 2018). The SIP signals are sensitive to changes in surface ion density and solid/liquid interfacial properties (Hao et al., 2015). The principle of SIP can be summarized briefly as follows.

When an external alternating electrical field is applied to a porous medium, the polar components in the medium, including the dipoles in the pore fluid, diffuse layer clouds near the solid–liquid interface, and the adsorbed ions at the stern layer of the solid interfaces will realign themselves towards new equilibrium states. Responses of the clusters of those polar components can be expressed in terms of complex conductivity, which is derived from the measured magnitude, |σ|, and phase shift, φ, of the porous media sample, and is expressed as

$$\begin{array}{c}{\sigma }^{*}=\left|\sigma \right|e^{i\phi }=\sigma \prime +i\sigma ″\hfill \\ \sigma \prime =\left|\sigma \right|*\cos (\phi )\hfill \\ \sigma ″=\left|\sigma \right|*\sin (\phi ),\hfill \end{array}$$

where $\sigma \prime$ is the real (conductive) part of the complex conductivity indicating the charge transport and energy losses, $\sigma ″$ is the imaginary (polarization) part of the complex conductivity indicating the charge polarization and energy storage, and i = $\sqrt{-1}$.

Schwarz (1962) developed a mechanistic model to describe the stern layer polarization theory, which relates the single Debye relaxation time, τ, and the characteristic pore or grain size, R, which is expressed as

$$\tau =\frac{R^2}{2\mu kT}=\frac{R^2}{2D},$$

where k is Boltzmann’s constant, T is the absolute temperature, μ is the surface ionic mobility of counter ions in the stern layer, and D is the diffusion coefficient of counter ions in the stern layer.

To date, there are limited SIP studies on AC and biochar (Gao et al., 2017, Haegel et al., 2012, Ntarlagiannis et al., 2017). Ntarlagiannis et al. (2017) investigated the SIP response of biochar remediation for a textile wastewater process, and their early promising results indicated the sensitivity of SIP signals for biochar; however, further tests are needed to examine the possibility of the SIP monitoring remediation process.

This study focused on investigating the remediation mechanisms of AC and biochar on Pb, as evaluated by the SIP method and physical and chemical characterization tools. There were two objectives of this work: (1) to study the differences in the mechanisms between AC and biochar on immobilization of Pb in contaminated water and (2) to determine whether SIP signals would be different during the Pb immobilization process using AC and biochar, and to investigate if the SIP technique is an effective tool for elucidating the mechanisms.