Abstract
This research investigated the removal of heavy metal ions (Cd, Cu, Pb, and Zn) and metalloid (As) common to stormwater runoff onto biochar-based media arranged in multiple configurations. Laboratory scale column experiments were conducted to quantify heavy metal removal efficiencies using sand, biochar, and nZVI-modified biochar (BC-nZVI) in four media configurations: a homogeneous mixture of sand and biochar (BCM); biochar layered in sand (BCL); BC-nZVI layered in sand (BCZ); and sand as a control. An inverse modeling approach was implemented to measured moisture and experimental data to determine media hydraulic parameters (θr, θs, α, n and Ks) and adsorption coefficients. The experiment was conducted using laboratory synthesized stormwater over 200 days at a rate of 5 cm/day. BCZ exhibited an excellent removal (99%) of As due to the high attachment to nZVI, via surface complexations. Biochar with abundant surface oxygen functional groups exhibited a great (99%) removal of Cd and Zn in both BCL and BCM columns. Water contents were observed 66.0, 44.3, 41.4, and 7.2% for BCL, BCM, BCZ, and sand, respectively. The attachment coefficients varied from 21.5 to 44.9, 16.1 to 19.3, 18.8 to 26.0, and 9.6 to 19.9 L/kg for BCL, BCM, BCZ, and sand, respectively. This study’s output provides useful information for stormwater management practices.
Highlights
The expansion of urban regions increases impervious surface area and has the potential to impact water environments
We examined retention capabilities of heavy metals such as Cd, Cu, Pb, and Zn, and a metalloid (As) along with hydraulic properties of commercially available biochar distributed in sand, nanoscale zero-valent iron (nZVI) modified biochar (BC-nZVI), and sand as a control
An energy-dispersive X-ray spectroscopy (EDS) site mapping (Figure 3) confirmed the presence of nZVI, and the porous areas of biochar seemed to have an agglomeration of nZVI, possibly due to the nanoparticles attaching and accumulating to the rough edges
Summary
The expansion of urban regions increases impervious surface area and has the potential to impact water environments. Hydrologic changes associated with urban growth are increased runoff peak, runoff volume, and reduced time to peak [1]. Changes in characteristics of urban runoff are not the only effects of urbanization; water quality is impacted. Urban runoff contains various amounts of metals, organic compounds, suspended solids, and inorganic compounds. During rainfall and snowmelt events, metals are transported to nearby surface waters, affecting the health of the ecosystem and contaminating groundwater. Heavy metals are known to cause health issues in humans and can enter the body by inhalation, skin absorption, and digestion. Health problems associated with heavy metal exposure include respiratory system damage, nervous system damage, and cancer [4]. Established treatment methods of urban runoff include liquid extraction, precipitation, reverse osmosis, and ion exchange, methods which achieve acceptable discharge concentrations; they are not always economical or technically feasible [5]
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