Batch investigations on P immobilization from wastewaters and sediment using natural calcium rich sepiolite as a reactive material

Highlights

• Natural calcium-rich sepiolite was calcined to enhance phosphorus removal capacity.

• 900 °C calcined calcium-rich sepiolite exhibit excellent sorption performance.

• Sorption was not influenced by pH, competing anions (except ${{\text{HCO}}_3}^{-}$) and humic acid.

• Calcined sepiolite could transform sediment reactive phosphorus into inter forms.

• Phosphate removed was primarily as Ca₈-P and Ca₁₀-P precipitation.

Abstract

Phosphorus from wastewaters and sediment flux to surface water represents a major source of lake eutrophication. Active filtration and in situ capping (which refers to placement of a covering or cap over an in-situ deposit of contaminated sediment) are widely used as a means to immobilize phosphorus from wastewaters and sediment, to mitigate lake eutrophication. There is, however, a need to develop more efficient means of immobilizing phosphorus through the development of binding agents. In this study, natural calcium-rich sepiolite (NCSP) was calcined at a range of temperatures, to enhance its phosphorus removal capacity. Batch studies showed that the 900 °C calcinated NCSP (NCSP900) exhibited excellent sorption performance, attaining a phosphorus removal efficiency of 80.0%–99.9% in the range of 0.05 mg/L–800 mg/L phosphorus concentrations with a dosage of 20 g/L. The material displayed rapid sorption rate (maximum amount of 99.9% of phosphate removal with 5 min) and could lower the very high phosphate concentration (200 mg/L) to less than 0.1 mg/L after 4 h adsorption. It was also noted that factors such as pH, competing anions (except ${{\text{HCO}}_3}^{-}$) and humic acid, had no effect on phosphorus removal capacity. The sediment immobilization experiment indicated that NCSP900 had the capacity to transform reactive phosphorus into inert-phosphorus and significantly reduce the amount of algal-bioavailable phosphorus. The excellent phosphorus binding performance of NCSP900 was mainly due to the improvement of point of zero charge (pH_PZC) as well as the transformation of the inert-calcium of NCSP to active free CaO during calcination. Phosphorus speciation indicated that phosphorus was mainly captured by relatively stable calcium-bound phosphorus (Ca-P) precipitation, which can account for 80.1% of the total phosphorus. This study showed that NCSP900 could be used as an efficient binding agent for the sequestration of phosphorus from wastewaters and sediment.

Introduction

Eutrophication has now become a global environmental problem attracting the attention of scientists and governments around the world. Persistent algal bloom in lakes and costal zones has caused major ecological, economic and social problems. Measures must therefore be taken to improve aquatic ecosystems that have become degraded due to excessively high phosphorus levels. Phosphorus is considered to be the limiting nutrient for the growth algae in freshwater ecosystems. Superfluous phosphorus input into freshwater ecosystems can induce eutrophication and subsequently result in the formation of harmful algal blooms. It thus becomes necessary to reduce the concentrations of phosphorus in receiving water bodies. In certain freshwater ecosystems, phosphorus originates mainly from external loading, such as industrial wastewaters, domestic sewerage and agricultural runoff, as well as internal loading from sediment flux (Xiong and Peng, 2008).

Technologies, such as chemical precipitation and biological treatment have been used to remove phosphorus from industrial, household and agricultural wastewaters (Sibrell et al., 2009). This typically requires considerable capital investment and costly maintenance (Kaasik et al., 2008; Sibrell et al., 2009). Active filtration through iron, aluminum or calcium-rich substrates to remove phosphorus from wastewaters has recently been considered an effective treatment method (Rentz et al., 2009; Kõiv et al., 2010). These substrates must be inexpensive, readily available, environment-friendly and efficient (Sibrell et al., 2009; Kõiv et al., 2010). To date, numerous reactive media have been developed and applied in practice. These include minerals (limestone, opoka, wollastonite, bauxite and zeolites), soils (laterite and marl), industrial byproducts (fly ash, red mud, burnt oil shale and slag materials) and man-made products (lightweight aggregates) (Johansson Westholm, 2006; Cucarella and Renman, 2009; Vohla et al., 2011). In summary, it seems that the calcium-rich materials are the most promising and widely-accepted reactive media used for wastewater treatment (Johansson Westholm, 2006; Cucarella and Renman, 2009; Vohla et al., 2011). Phosphorus has been effectively removed from wastewater through absorption, but mainly through precipitation of chemically-stable phases (Kõiv et al., 2010; Renman and Renman, 2010; Claveau-Mallet et al., 2012). There are, however, some disadvantages associated with a number of sorbents currently in use, such as low phosphorus removal efficiency and the use of certain highly pH-dependent natural calcium-rich materials that are only suitable for a small number of applications (Yin et al., 2011a). There is thus a need to modify these promising sorbents in order to expand their sorption performance with respect to phosphate. Simple and easy methods have been encouraged so as to avoid additional costs. For example, the heating of calcium-rich media at high temperature is a common method used to enhance material performance in terms of phosphorus retention capacity. During calcinations, CaO will probably form, which has a more reactive Ca-phase than commonly-existing calcareous minerals such calcite and dolomite (Karaca et al., 2006; Vohla et al., 2011).

Sediment dredging and in situ active capping have been used to control internal loading of water bodies and eutrophication in lakes and estuarine waters (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011). There is also a need to develop an efficient and low-cost sediment capping agent that can effectively bind sediment phosphorus and hence suppress its release from sediments (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011). Conventional Ca/Fe-rich materials, which originated from industrial byproducts such as fly ash, red mud and slags, may not be suitable for ameliorating the effects of eutrophication regardless of their excellent phosphorus removal capacity and efficiency, since such substances may potentially have a toxic effect on aquatic species in lakes (Xiong and Peng, 2008). In contrast, clay minerals that are based on capping agents or adsorbents are normally environmental friendly and also inexpensive. They can thus be used safely in freshwater ecosystems (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011).

Sepiolite, a hydrated magnesium silicate clay mineral with a fibrous chain structure, is nontoxic and relatively inexpensive. The main deposits of sepiolite are located in Anatolia in Turkey, Ceelbuur in Somalia, South Central China and Spain with 70% of the world reserves and annual output being approximately 1,300,000 tons (Hrenovic et al., 2010). Structurally, sepiolite consists of a ribbon-like structure that alternates with open channels along fiber axes, which provides sepiolite with good adsorption properties (Hrenovic et al., 2010). In environmental studies, sepiolite has been widely used to absorb heavy metals (Kocaoba, 2009), chloride (González-Pradas et al., 2005), basic dyes (Tekbas et al., 2009) and cationic surfactants (Sabah et al., 2002). Approximately 10 million tons of sepiolite deposits exist in China (approximately 1/5 of the world's reserve) (Yin et al., 2011a). The chemical composition of sepiolite varies geographically and the sepiolite in China is characterized by high calcium content, which has been found to have a calcium oxide content in the range of 20.5%–27.1% (Yin et al., 2011a). Previous studies indicated that natural calcium-rich sepiolite (NCSP) from Nanyang (Henan Province) had a phosphorus removal capacity as high as 32.0 mg P/g at acid conditions (Yin et al., 2011a). This contrasts with certain characteristics of NCSP that greatly narrow its usage, such as its pH-dependent characteristics and low phosphorus removal efficiency at low concentrations. A proper activation method is therefore needed to enhance the phosphorus sorption performance of this compound. The objectives of this study were therefore as follows: (1) to test the effect of calcinations (or material pre-treatment) on its P removal capacities; (2) to test the reactive materials in batch experiments for their suitability in terms of immobilization capacity P in both wastewaters and sediment; and (3) to characterize the P retention mechanism on the reactive material. The above-mentioned research would represent a necessary pre-evaluation process for controlling lake eutrophication that results from phosphorus-contaminated water and sediment fluxing.