Interactions among low-molecular-weight organics, heavy metals, and Fe(III) during coagulation of landfill leachate nanofiltration concentrate

Highlights

• LMWOs were efficiently removed by specialistic adsorption of Fe₂(OH)₂⁴⁺, Fe₃(OH)₄⁵⁺.

• Humification played an important role on the coagulation removal of LMWOs.

• LMWOs dissociated due to acid effect and inner-sphere complexes.

• Acid effect and inner-sphere complexes decreased the coagulation removal of LMWOs.

• Partition of LMWO components showed various influences on heavy metals removal.

Abstract

The generation of landfill leachate nanofiltration concentrate (LLNC) has been a dilemma for leachate treatment plants because it contains large amounts of refractory organics with low molecular weight (LMWO), as well as heavy metals (HMs), and is difficult to handle. The coagulation removal of LMWOs is a significant challenge, as is the removal of HMs bonded to LMWOs. In this study, coagulation through the dosing of FeCl₃ was used to remove LMWOs and HMs from LLNC. The results interestingly demonstrated that the removal rates of dissolved organic carbon (DOC), Cr, Ni, and As reached up to 84.1% ± 3.9%, 91.0 ± 1.1%, 73.1 ± 2.2%, and 96.9 ± 1.5%, respectively. The partition of LMWO components, as well as the interactions among the LMWOs, HMs, and Fe(III) were investigated to determine the mechanism behind the LMWO and HM removal. LMWOs with a high degree of humification, including humic and fulvic acid-like components, were preferentially removed through aggregation and electrostatic attraction originating from the specialistic adsorption of Fe₂(OH)₂⁴⁺ and Fe₃(OH)₄⁵⁺. In addition to being removed, a portion of these two components was dissociated into aromatic protein I, aromatic protein II, and soluble microbial by-product-like materials due to an acid effect and the formation of inner-sphere complexes. A redundancy analysis revealed that As, Cr, and Ni are mainly removed through the electrostatic attraction of Fe(III), bonding to humic substances and hydrophilic organics, respectively. The outcomes provide a new understanding on the coagulation removal of LMWOs and HMs.

Introduction

Among various waste treatment methods, landfills are still the preferred method for waste management in most developing countries owing to their low-cost and easy operation (Ye et al., 2017, Chen et al., 2019). However, the generation of landfill leachate containing significant amounts of refractory organics, ammonia, and heavy metals (HMs) makes it a challenge to efficiently remove these pollutants through traditional techniques (Li et al., 1999, Ye et al., 2017). In China, a combined membrane filtration process of ultrafiltration-nanofiltration-reverse osmosis (UF-NF-RO) is predominantly used after biological treatment (Long et al., 2017) to guarantee that the effluent meets the strict discharge criteria of the Chinese standards (MEP and GAQSIQ, 2008). During the membrane process, large organic compounds can be efficiently retained by a UF membrane (Nilson and Digiano, 1996, Meylan et al., 2007). As a consequence, low-molecular-weight organics (LMWOs) were permeated and then concentrated in landfill leachate nanofiltration concentrate (LLNC) (Schäfer et al., 2000, Meylan et al., 2007). Moreover, an NF membrane also rejects polyvalent inorganic salts, and poisonous HMs can be simultaneously concentrated in LLNC (Wang et al., 1997). The high concentration of LMWOs and HMs in LLNC has resulted in a secondary pollution problem, and has thus become a dilemma for leachate treatment plants.

As a simple and cost-effective technique, coagulation is among the most frequently applied technologies for the removal of high-molecular-weight organics from wastewater (Ishak et al., 2018, Yusoff et al., 2018, Tripathy and Kumar, 2019, Webler et al., 2019), and has thus been widely used as an alternative method to reduce the treatment cost and guarantee the efficiency of advanced oxidation processes (Wang et al., 1997, Amor et al., 2015, Ishak et al., 2017, Ishak et al., 2018, Güneşa et al., 2019), mechanical vapor recompression (Ye et al., 2017), and biological treatment (Ansari et al., 2018). However, LMWOs tended to be more difficult to remove during the coagulation process because they were more hydrophilic (Edzwald, 1993, Zhao et al., 2009, Zhao et al., 2016, Xu et al., 2018). To date, several studies have investigated the coagulation performance for the removal of LMWOs from LLNC, and high removal rates of 75% (Huang et al., 2015) and 82% (Long et al., 2017) were obtained using polymeric aluminum chloride, and FeCl₃, respectively. Such high removal rates run contrary to conventional wisdom because it has been widely-accepted that LMWOs tended to be more difficult to remove during the coagulation process (Edzwald, 1993, Zhao et al., 2009, Zhao et al., 2016, Xu et al., 2018), and thus a deep study on the removal mechanism will provide a new understanding of the removal of LMWOs through conventional coagulants. Unfortunately, however, specific attention has not been paid to the partitioning of LMWO components, or to the interactions between the LMWOs and coagulants, which are essential to investigating the mechanism behind dissolved organic matter (DOM) removal through coagulation (Dia et al., 2017).

Previous studies (Rudd et al., 1984, Wang et al., 2003, Xu et al., 2016) have pointed out that various DOM components tend to intensively form complexes with HMs, and in view of the above, large organometallic complexes can be formed in LLNC. It can be inferred from this that coagulation technology has the potential to simultaneously reduce the HM complexing with organics. This needs to be studied carefully, and a method for controlling HM pollution in LLNC should be developed. The present study therefore aims to understand the coagulation removal of LMWOs and HMs from LLNC in a more comprehensive manner. First, the coagulation performance on the removal of LMWOs and HMs from LLNC was examined. Next, the aggregation and dissociation of different LMWO components, the main ferric species, and organofunctional groups contributing to LMWO removal were analyzed. Finally, the interaction among coagulant/LMWOs and HMs was investigated.