Hydrated lanthanum oxide-modified diatomite as highly efficient adsorbent for low-concentration phosphate removal from secondary effluents

Highlights

• La-diatomite possessed a high removal efficiency to low-concentration phosphate.

• La-diatomite exhibited the maximum adsorption capacity of 58.7 mg P/g.

• Phosphate adsorption was a pH-independent process.

• La-diatomite showed superior treatment capacity for simulated secondary effluents.

• The underlying adsorption mechanisms were elucidated in detail.

Abstract

The requirement to the phosphorus (P) emission from wastewater treatment plants (WWTPs) is becoming increasingly strict, which makes an advanced treatment for the low-concentration phosphate removal from secondary effluents indispensable. In present work, hydrated lanthanum (La) oxide-modified diatomite composites (La-diatomite) were fabricated by a facile method and employed as the highly efficient adsorbent for the low-concentration phosphate removal from simulating secondary effluents. Comparative experiments indicated that the La-diatomite treated by 0.1 mol/L LaCl₃ exhibited the highest La availability (P/La molar ratio of 2.30) and performed good selectivity to phosphate adsorption even with the coexistence of competing anions and humic acid. The maximum P adsorption capacity reached to 58.7 mg P/g and the 96% P was removed quickly within 30 min at initial phosphate concentration 2 mg P/L. Insignificant La leaching was observed during the process due to the La stabilization by macroporous diatomite. Eight cycles of adsorption-desorption experiments revealed that the excellent repeated use property of La-diatomite. At the column test, La-diatomite showed superior treatment capacities of 3455 kg water/kg La-diatomite for simulated secondary effluents. The La-diatomite maintained high and stable adsorption effectiveness in wide pH range, which should be attributed to the synergistic effect of electrostatic interactions, ligand exchange and Lewis acid-based interaction. This work might provide a candidate for low-concentration phosphate removal from secondary effluent to alleviate the eutrophication.

Introduction

Phosphorus (P) is an essential nutrient in aquatic ecosystems (Sun et al., 2017; Wang et al., 2012; Xu et al., 2018a), but excessive P discharge can trigger severe eutrophication and threaten ecosystem and human healthy (Conley and Likens, 2009; Mayer et al., 2016; Shang et al., 2017). Therefore, the increasingly strict requirements to the P emission have been explicitly stipulated to control water pollution (Jung et al., 2014; Xie et al., 2016). Specifically, the current maximum allowable P is set as 0.5 mg P/L in China, replacing the previous 1.0 mg P/L in secondary effluents from wastewater treatment plants (WWTPs). Though traditional P treatment technologies like biological process are widely employed in removing P at WWTPs, to our knowledge, there are no messages about whether the biological technology alone is efficient with high removal rates of above 80% and whether the attainable P concentrations in effluents are adequately low to alleviate eutrophication (Chouyyok et al., 2010; Rott et al., 2018). Hence, it requires further phosphate treatment for the secondary effluent to get satisfactory low levels of phosphate.

Adsorption is a promising method to remove P due to its advantages of operational simplicity, environmental-friendliness, higher effectiveness even at low phosphate concentrations (Cao et al., 2018; Mayer et al., 2016; Xiong et al., 2017). Recently, metal-based adsorbents have been extensively investigated to adsorb phosphate anions in wastewater, such as magnesium oxide-enriched biochar (Zhang et al., 2012b), hydrous zirconia-modified zeolite (Fan et al., 2017) and hydroxy-iron-aluminum pillared bentonites (Yan et al., 2010). Most of these adsorbents exhibit high adsorption capacity and selectivity to phosphate. Nevertheless, these studies mainly focused on relatively high initial P concentration of above 20 mg/L, which are not very representative for the wastewater quality in WWTPs. Generally, the P concentration of WWTPs treating domestic wastewater is below 10 mg P/L (Rott et al., 2018). Thus, it is of great significance and imperative to exploit the adsorbents with higher adsorption capacity at relatively low initial P concentration.

Previous studies have shown that the binding capacity between metal and phosphate anions controls the phosphate adsorption by metal-based adsorbents (Huang et al., 2014a, 2014b). Considering the strong specific affinity between Lanthanum (La) and phosphate even at trace level (solubility product of La phosphate pK = 26.16) (Dong et al., 2017; Huang et al., 2014b), La could be dispersed onto the support to perform the specific interaction for low-concentration phosphate adsorption. Interestingly, it has been pointed out that the characterize of support materials has a great impact on the leaching of loaded La, which is vital for the P removal efficiency of La-based materials (Liu et al., 2013; Xie et al., 2014b). Furthermore, studies have shown that the support materials with macropores (average pore diameters > 50 nm) can enhance the accessibility of La to phosphate adsorption and further promote the crystallization of La phosphate, an indispensable step for P complexation by La, by effectively alleviating pore blockage (Huang et al., 2014a; Yang et al., 2012). All these findings demonstrate that a proper porous support material loaded La is able to adequately remove phosphate at low initial concentration. In this study, the diatomite is chosen as the promising natural support material to immobilize La because of its unique characteristics of macroporous structure, high porosity (25–62%) (Aivalioti et al., 2012; Jiang et al., 2016). Diatomite with proper modification has been widely applied in the removal of heavy metal ions and organic substances from wastewater (He et al., 2017a; Yuan et al., 2010). However, its adsorption capacity to phosphate anions is poor due to the fact that its surface has negative charge at neutral pH (Xie et al., 2014a). Negatively charged surface is not beneficial to attract H₂PO₄⁻ or HPO₄²⁻ in terms of electrostatic interaction. Thus, the specific modification is necessary to transform the surface charge of natural diatomite from negative to positive, which will improve its performance to adsorb phosphate anions. Owing to the diatomite's characteristic structure and the strong binding ability between La and phosphate, the diatomite modified with La species might be efficient adsorbent to remove the low-concentration phosphate in application.

Herein, the hydrous lanthanum (La) oxide/diatomite composites (La-diatomite) were synthesized in situ for phosphate removal from simulating secondary effluents containing relatively low-concentration phosphate. The equilibrium properties and kinetics of phosphate adsorption by La-diatomite were investigated in batch experiments. Continuous adsorption column experiments were conducted to assess the practical performance and stability of La-diatomite. Finally, the underlying adsorption mechanisms were elucidated through the effects of pH and the characterization including zeta potential, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).