Remediation of contaminated urban river sediment based to iron-rich substrate: A comparative study with chemical oxidants

Abstract

In situ sediment remediation is commonly used for controlling the release of pollutants (N, P, C and S) from polluted urban river sediments. The addition of oxidants can effectively solve the odour​ problem and control the conversion of sulphur in sediments. Laboratory experiments were established to explore the effect of improving contaminated urban river sediment quality with hydrogen peroxide, calcium nitrate, calcium hypochlorite and iron-based substrate (IRS). The results verified that the redox potential increased to 161 mV and 90 mV after 30 days of calcium hypochlorite and calcium nitrate treatment, respectively. The addition of calcium hypochlorite or calcium nitrate reached 97.9% and 94.8% high removal efficiency of acid volatile sulphide due to the oxidation. The addition of IRS decreased the ammonia and total organic carbon concentrations, indicating that the IRS were conducive to controlling the release of 73.2% of the total nitrogen (TN) from river sediment. The consequence also showed that compared with other oxidants, IRS had remarkable influences on the interstitial sulphate, iron and nitrate contents in the sediment, clearly showing comprehensive application prospects.

Introduction

Incomplete treatment of industrial wastewater, domestic wastewater, agricultural and rainwater runoff has severely destroyed water quality of urban river for many years (Lian et al., 2008). Large amount of contaminants discharged into rivers has also increased obviously with rapid growth of the society and economy. The continuous growth of contamination and exceeded pollutants (N, P, C, and S) causes the rivers to become odorous and turbid beyond their carrying capacity (Islam et al., 2015). Contaminants contained in static water bodies with low flow rate are difficult to diffuse, which is prone to water pollution. Hence, the expansion of urban river contamination is progressively becoming the greatest threats to China’s aquatic ecosystems (Gao et al., 2014, Jian et al., 2014).

Organic contaminants from effluent massively deposited in polluted river sediment, and their excessive degradation consumes a large amount of dissolved oxygen (DO), hydrogen sulphide (${\text{H}}_2$S) by microbial reduction in a low redox potential [oxidation–reduction potential (ORP)] due to low DO (Berner, 1970). The decreasing ORP of sediment lead to reduction of Fe and Mn (Miao et al., 2006, Schauser et al., 2010). Afterwards, reduced Fe(II) in the sediment and overlying water combines ${\text{H}}_2$S to iron sulphide (FeS) having low solubility meanwhile adhered to the organic particles, then the blackening of the sediment and water happened. Moreover, toxic contaminant as ammonia (NH${}_4{}^{+}$-N), ${\text{H}}_2$S, and other odorous gaseous mixtures leads to odorous phenomena which are the products of anaerobic respiration (Net et al., 2015). Urban river contamination can be improved with advanced treatment of wastewater and mitigation of external pollution sources (Chakraborty et al., 2014). However, large amounts of pollutants (N, P, C and S) can be continuously released from sediment to the water body, thereby postponing the environmental restoration of eutrophic lakes (Wang et al., 2017). In addition, as an important internal source of nutrients, the concentration of pollutants (N, P, C and S) in sediment pore water is much higher than that of overlying water, so the change of nutrient concentration in sediments greatly affects the overlying water quality. For this reason, the remediation of contaminated sediments and the improvement is much slower than overlying water. Some studies have confirmed that the addition of chemical oxidants {[i.e., calcium hypochlorite[Ca(ClO)₂], calcium nitrate[Ca(NO₃)₂], hydrogen peroxide (H₂O₂)} to river sediments can effectively improve eutrophic ecosystems (Bakker et al., 2016, Liu et al., 2016a, Yin and Kong, 2015), which can accelerate the nitrogen removal activity and meanwhile improve the oxidation conditions (Shimizu and Nakano, 2009). Under the action of these oxides, S${}^{2-}$ and Fe^2＋ were transformed to SO${}_4{}^{2-}$ and Fe^3＋, which modify the colour in black sediment. The conversion of NH${}_4{}^{+}$-N to ${\text{N}}_2$ due to electron transport by strong oxidizing substances also reduces the odour. Remediation purposes has been achieved by chemical oxidation as infusing calcium nitrate into river sediment to decreasing phosphorus release and limit these flowing algae in the eutrophic water (Glendell and Brazier, 2014, Perks et al., 2015). Furthermore, nitrate can abate odour from sediment induced by sulphate-reducing bacteria (Jiang et al., 2009). Partial organic compounds have been used to decrease SO${}_4{}^{2-}$ to S${}^{2-}$ when nitrate is metabolized (Jefferson et al., 2002).

Acid volatile sulphide (AVS) is the origin of sulphur-containing odour as ${\text{H}}_2$S, and are closely related to the colour intensity of the sediments, thus directly reflecting the degree of contamination of organically enriched sediments (Berner, 1964, Wilson and Vopel, 2012). Therefore, it is an urgent research topic to reduce the AVS concentration in black odour deposits and control the release of sulphur-related odours in urban rivers.

The iron-rich substrate (IRS) based on iron–carbon micro-electrolysis and composed of iron scraps and activated carbon was developed and applied. According our previous study, IRS showed efficient nitrogen and phosphorus removal function (Deng et al., 2017, Hu et al., 2019). Moreover, the [H] and O$\cdot$ generated at the iron–carbon micro-electrolysis cathode have strong chemical activities (Yang et al., 2017). Thus, it is feasible to use IRS to remediate contaminated urban river sediment.

In this study, we studied the process and mechanism of catalytic Fe–C micro-electrolysis applied for remediation of black and odorous sediments. Compared with the traditional chemical oxidants, the Fe–C micro-electrolysis reaction in the iron-rich substrate can directly remove nitrogen and phosphorus in the sediment and water. Meanwhile, as a biofilm carrier, the iron-rich substrate can provide electron donors (Fe${}^{2+}$) for the denitrification process under the condition of micro-oxygen, so as to realize simultaneous nitrification and denitrification. A comparative study of iron-rich substrate and chemical oxidants (i.e., Ca(ClO)₂, Ca(NO₃)₂, H₂O₂) on remediation of contaminated urban river sediment, concerning with the variation in AVS and Fe(II), has been investigated.