Competitive adsorption of phosphate and dissolved organic carbon on lanthanum modified zeolite

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Highlights

  • Competitive adsorption of P and DOC by lanthanum modified zeolite was studied.

  • The maximum adsorption capacity of P and DOC reached 52.25 mgP/g and 41.32 mgC/g.

  • DOC did not affect P adsorption but P greatly reduced DOC adsorption.

  • Adsorption sites for ligand exchange had higher affinity for P than for DOC.

  • DOC can be additionally adsorbed by uncharged hydroxyls via hydrogen bonding.

Abstract

Lanthanum (hydr)oxide-based materials are attractive as highly efficient adsorbents for phosphate removal from both sewage and lake environment. However, dissolved organic carbon (DOC) coexists in the waters and exact information is still lacking on how DOC influence the phosphate adsorption process. In this study, competitive adsorption of phosphate and DOC on lanthanum modified zeolite (LMZ) was investigated using humic acid as the representative. In LMZ, lanthanum hydroxide was shown to be the active ingredient accounting for >98% of the binding sites of both phosphate and DOC. Without competition, the maximum adsorption capacity of phosphate and DOC estimated from the Langmuir isotherm model was 52.25 and 41.32 mg/g, respectively. When coexisted, DOC did not affect the adsorption of phosphate while phosphate reduced the adsorption of DOC by ~40%. In addition, preloading LMZ with DOC had little effect on phosphate adsorption while coating with phosphate substantially lowered DOC adsorption. Furthermore, phosphate can release most of the adsorbed DOC (>60%), while DOC can not replace adsorbed phosphate (<2%). The adsorption kinetics of both phosphate and DOC was best described by the psudo-second-order model (r2 > 0.999). The adsorption of both phosphate and DOC increased with decreasing pH or increasing ionic strength. We proposed that phosphate was competitive than DOC for the ligand exchange sites of singly-coordinated hydroxyls, but DOC can be solely adsorbed onto the uncharged hydroxyls via hydrogen bonding.

Introduction

Eutrophication leads to lake quality degradation manifested in algal blooms, low dissolved oxygen, fish kills, increased sediment accumulation rates, low aesthetic value, and species shifts of both flora and fauna [1], [2], [3]. Eutrophication control may be implemented through phosphorus control because overenrichment of phosphorus is widely recognized as the primary cause of eutrophication in relatively stagnant inland waters and, for practical purposes, phosphorus is also the nutrient that can be effectively reduced to the desired concentration [4], [5], [6].

Phosphorus control requires both the reduction of the external phosphorus inputs and the in-lake phosphorus control [4], [7], [8]. In the latter case, the removal of phosphorus from water column and the retardation of phosphorus release from the sediment are essential [9], [10].

Lanthanum (La) is one of the few elements that have very high affinity for phosphate [11]. Within the group of rare earth elements, La is also among the most abundant. The average crustal abundance of La (38 mg/g) is similar to elements such as copper (Cu; 50 mg/g) and other elements like cobalt (Co; 23 mg/g), and lead (Pb; 20 mg/g) [12]. Similarly to other metal (hydr)oxides which are known good phosphate adsorbents [10], [13], a number of adsorbents containing lanthanum (hydr)oxide have been intensively investigated to remove phosphate from water/wastewater in recent years [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. These adsorbents exhibited excellent adsorption performance in terms of high adsorption capacity, rapid adsorption kinetics, good selectivity and applicability over a relatively wide pH range. In recent years, lanthanum (lanthanum hydroxide)-modified zeolite (LMZ) that was developed from coal fly ash via a one-pot process in our laboratory was shown to be very effective to reduce phosphorus concentration in lake water and hinder the phosphorus release from sediment under different pH, temperature and anaerobic conditions [29], [30], [31].

Dissolved organic carbon (DOC) exists ubiquitously in water environment including wastewater, natural surface water [32], [33], [34], and groundwater [35]. Humic substances such as humic acid (HA) and fulvic acid (FA) form the major fraction of DOC in aquatic systems [36], [37]. Due to their polycarboxylic nature, humic substances may be adsorbed by aluminium and iron (hydr)oxides [38], [39], [40], [41]. Ligand-exchange mechanism between carboxylic/phenolic functional groups and the metal (hydr)oxide surface is the dominant mechanism [40], [41]. Since ligand-exchange mechanism is generally responsible for phosphate adsorption, competition between phosphate and humic substances for adsorption by iron or aluminum (hydr)oxides is well recognized. Hence, interference of phosphate binding capacity of lanthanum (hydr)oxide-based adsorbents by DOC is suspected and thus has been investigated [14], [16], [18], [25], [27]. However, contradictory results emerge from the literatures. Specifically, while some studies showed a substantial decrease of adsorbed phosphate in the presence of HA [14], [16], [27], other investigations found that the phosphate adsorption was nearly independent of the presence of HA [18], [25]. These previous studies focused on the development of novel La modified materials and the removal efficiency and mechanism of phosphate. Because there has been little systematic work to elucidate the influence of DOC on phosphate adsorption by lanthanum (hydr)oxide-based materials to date, exact information is still lacking. Furthermore, though the adsorption behavior of DOC is crucial to better understand the competition of DOC for phosphate adsorption, investigation about DOC adsorption by lanthanum (hydr)oxide-based adsorbents has not been carried out to our best knowledge. Understanding how DOC influence phosphate adsorption is obviously essential to promote the application of the novel adsorbents containing lanthanum (hydr)oxide in phosphate removal from wastewater/lake water.

The aim of our present study was to investigate the competitive adsorption of phosphate and DOC on LMZ in detail. To this end, separate and simultaneous adsorption of phosphate and DOC and the adsorption of phosphate by DOC-coated LMZ as well as the adsorption of DOC by phosphate-coated LMZ were carried out. In addition, the effects of time, pH and ionic strength on the adsorption of phosphate and DOC were examined.

Section snippets

Materials

LMZ is a powder product developed in our laboratory and was obtained by the one-pot method using coal fly ash as the raw material. The type of zeolite in LMZ was NaP1 (Na6Al6Si10O32·12H2O, JCPDS code 39-0219). Lanthanum hydroxide was loaded on the surface of zeolite grains. The La2O3 content in LMZ was determined to be 28.8%. More characterization of this material is in papers [28], [29]. For comparison purposes, zeolite before lanthanum modification was also obtained using the same coal fly

Separate and simultaneous adsorption of phosphate and DOC

The separate adsorption isotherms of phosphate and DOC on LMZ (Fig. 1) showed that, at low concentrations, both phosphate and DOC were greatly adsorbed so that the initial part of the curves are nearly vertical. These isotherms resembled a H-curve isotherm, which is indicative of a very high affinity for solutes [43]. Compared with the great adsorption capacity of both phosphate and DOC by LMZ, those by zeolite without lanthanum hydroxide, which is the precursor of LMZ, were trivial. The

Discussion

Our experimental results indicated that both phosphate and DOC can be adsorbed by LMZ and the fraction which is responsible for adsorption was the lanthanum hydroxide, since zeolite without lanthanum modification (the precursor of LMZ) showed very trivial adsorptive ability (~1% of that for LMZ). The C/P molar ratio of the maximum adsorption capacity for phosphate and DOC was about 2.1. Considering the high physical volume occupation of DOC when compared with the small inorganic phosphate

Conclusions

LMZ, which was obtained by the one-pot method using coal fly ash as the raw material, was previously shown to be a promising material for phosphorus inactivation in lake sediments. But lake water and pore water in lake sediments contain DOC compounds like HA. Our present study showed that LMZ can adsorb both phosphate and DOC, with the Langmuir maximum adsorption capacity of 52.25 mgP/g and 41.32 mgC/g, respectively. Results from simultaneous adsorption of phosphate and DOC and the adsorption

CRediT authorship contribution statement

Xiaodi Li: Conceptualization, Investigation, Writing - original draft. Yue Kuang: Visualization, Methodology. Jiabin Chen: Validation. Deyi Wu: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the Ministry of Science and Technology of China (2012ZX07105002-03) and the National Natural Science Foundation Project of China (21507084).

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