Preliminary Evaluation of Biological and Physical–Chemical Chromium Removal Mechanisms in Gravel Media Used in Constructed Wetlands

Abstract

Constructed wetlands have been shown to achieve high chromium and organic matter removal efficiencies when treating tannery wastewaters. Further, findings suggested chromium was potentially binding to iron oxides and microbial surfaces on the wetland media. The purpose of the present study was to distinguish between physical-chemical and biologically mediated removal mechanisms operating on iron-containing media. A total of 12 small-scale reactors were used for testing three conditions: biotic, abiotic aerobic and abiotic anaerobic. All systems were operated in 3-day batches, with the biotic and abiotic aerobic systems operating for 11 batches and the abiotic anaerobic operating for five batches. The results show that biotic systems achieved significantly higher chromium removal efficiencies (83%) than both abiotic treatments (16%). Biotic reactors quickly lowered dissolved oxygen concentrations, removing an average of 53% of the influent organic matter in the process. Redox conditions were affected by microbial metabolism, favouring iron release from the media surface. Findings suggest that microbiological activity catalyses chromium removal from tannery wastewaters. Chromium removals achieved in sterile reactors illustrate that the media influences metal retention in constructed wetlands; however, microbial interactions with both the media and tannery effluent achieve overall greater chromium removal than achieved solely by physical-chemical mechanisms.

Chromium removal in constructed wetlands: A review

“…Studies have shown that gravel media can remove more Cr than plant vegetation (Dotro et al, 2011a). Stefanakis and Tsihrintzis (2012) reported that the gravel layer plays a vital role in the Cr removal process.…”

“…According to Dotro et al (2009), in gravel (media) Cr can be present within the biofilm or as iron oxides in a sorption process. Dotro et al (2009Dotro et al ( , 2011a, conducted experiments in laboratory-scale CWs to investigate Cr retention in gravel beds and suggested that porous media containing iron in combination with biological activity can positively? influence Cr removal efficiency.…”

“…The researchers conclude that in this case Cr was removed by sorption. In a previous study (Dotro et al, 2011a), used gravel media and achieved 83% Cr removal efficiency.…”

“…CWs have been used to treat municipal wastewaters (Vymazal, 2005), acid mine drainage (Machemer et al, 1993;Mays and Edwards, 2001;Yang et al, 2006), industrial wastewaters Khan et al, 2009;Di Luca et al, 2011;Kongroy et al, 2012), agricultural wastewaters (Bubba et al, 2004;Nahlik and Mitsch, 2006;Grafias et al, 2010) and tannery wastewaters (Kucuk et al, 2003;Calheiros et al, 2007Calheiros et al, , 2008aCalheiros et al, , b, 2010Calheiros et al, , 2012Dotro et al, 2011a). These systems are very cost effective compared to conventional wastewater treatment methods (Knox et al, 2006).…”

“…Various applications/experiments using CWs for Cr removal have been recorded in the last decade, at both laboratory-scale (Srisatit and Sengsai, 2003;Maine et al, 2006;Mant et al, 2006;Liu et al, 2010;Yadav et al, 2010;Dotro et al, 2011aDotro et al, , b, 2012 and full-scale (Vymazal, 2005;Maine et al, 2007;Khan et al, 2009;Hadad et al, 2010;Di Luca et al, 2011;Kelvin and Tole, 2011). These applications either examine Cr removal from various types of wastewaters (Kucuk et al, 2003;Vymazal, 2005;Mant et al, 2006;Maine et al, 2006Maine et al, , 2007aMaine et al, , 2009Lesage et al, 2007a;Aguilar et al, 2008;Dorman et al, 2009;Khan et al, 2009;Arroyo et al, 2010;Hadad et al, 2010;Di Luca et al, 2011;Kelvin and Tole, 2011;Dotro et al, 2011bDotro et al, , 2012Baker et al, 2012;Fibbi et al, 2012;Kongroy et al, 2012;Soda et al, 2012) and activated sludge (Stefanakis and Tsihrintzis, 2012) or from Cr-containing aqueous solutions Yadav et al, 2010;Michailides et al, 2013;Sultana et al, 2014).…”