Phosphorus removal from aqueous solution using Al-modified Pisha sandstone

Abstract

Pisha sandstone (PS) can increase the ability of sandy soil to retain water and some nutrients due to the presence of montmorillonite, but its inadequate phosphorus adsorption capacity affects its phosphorus holding capacity. This study aims to determine whether Al modification can enhance the phosphate adsorption capacity of Pisha sandstone and to identify the adsorption mechanism. Batch adsorption kinetic and equilibrium isotherm experiments were conducted to investigate the phosphate adsorption behavior of Al-PS (Pisha sandstone modified with Al). The results showed that PS could be successfully modified with Al. As the Al modification ratio increased, the P adsorption capacities of Al-PS increased and the optimal Al-to-PS ratio was 0.81:1. Considering the modification cost and P removal efficiency, it was decided to apply 0.27:1 the Al-to-PS mass ratio in batch adsorption experiments and characterization. The phosphate adsorption capacity decreased but the removal rate increased with increasing amount of Al-PS. The temperature had no significant effect on the adsorption, and the adsorption kinetic and isothermal adsorption data could be well fitted by the pseudo-second-order and Temkin models, respectively. The phosphate adsorption mechanism of Al-PS for phosphate was proposed to be electrostatic adsorption. This study suggested that Al-PS could be considered to be an effective material for phosphate adsorption, and it could enhance the soil phosphorus content when added into sandy soil.

Introduction

In the border region between Shanxi, Shaanxi and Inner Mongolia within Northwest China (total area of 5.44 × 10⁴ km²), the main soil types are aeolian sandy soil and loess soil, and Pisha sandstone can be found in 1/3 of this area (Zhen et al., 2016), as shown in Fig. 1. Pisha sandstone (also known as “feldspathic sandstone” (Han et al., 2012a), “soft rock” (Sun and Han, 2018), and “montmorillonite-enriched sandstone” (Jia et al., 2019)) is a kind of sandstone interbedded with thick sandstone, sandy shale and shale, found in the Permian, Triassic, Jurassic, and Cretaceous strata (Zhen et al., 2016). It can be as hard as stones when dry and as soft as mud when wet, and these properties result in the severe erosion of water and soil in this area. As a result, it is the main source of coarse sediment in the upper and middle reaches of the Yellow River and its soil erosion rate is approximately 20,000 t/(km²·a), making this area “the most severe soil eroded area in the world” (Ma and Zhang, 2016). A study showed that the main mineral composition of Pisha sandstone includes quartz, montmorillonite and calcite, and the content of montmorillonite is as high as 30% (Wang et al., 2013). Due to the high content of montmorillonite, Pisha sandstone has a relatively high cation exchange capacity (CEC) and thus a certain adsorption capacity. As an inexpensive and easily obtained material, Pisha sandstone could be utilized as a resource to reduce its hazards and broaden its application. Therefore, studying the resource utilization of Pisha sandstone is worthwhile. A study has proved that incorporating Pisha sandstone into sandy soil can significantly increase soil water retention (She et al., 2014). Over 1600 hm² of new arable farmland can be produced by mixing Pisha sandstone with sandy soil (Han et al., 2012b). To date, Pisha sandstone has been studied as a topsoil for mine land reclamation (Jia et al., 2019), a geopolymer cement for soil and water conservation (Dong et al., 2014), a cementitious material for construction (Li et al., 2018), and an adsorbent for pollutants such as Cd(II) and Cu(II) (Wang et al., 2020), Pb(II) (Wen et al., 2014), phosphate (She et al., 2015a) and ammonium nitrogen (She et al., 2015b). However, although it has shown acceptable adsorption capacities for heavy metals and ammonium nitrogen, the adsorption ability for phosphorus is not too high.

It is well known that phosphorus is an essential element for plant growth (Orhan et al., 2006), but it also causes eutrophication. Currently, approaches for removing phosphorus from water include biological (Yuan et al., 2012), chemical (Kim et al., 2015), and adsorption methods (Qiao et al., 2019). Of these approaches, the adsorption method is widely used due to its high efficiency, recyclability, simple operation and low sludge yields (Peng et al., 2020). Materials studied for phosphorus removal include biochars (Yao et al., 2013a), clay minerals (Fang et al., 2017), and metal oxides (Li et al., 2016). However, the phosphate adsorption performance of raw adsorbent materials, such as biochars (Liao et al., 2018) and natural clay minerals (Huang et al., 2014) is poor. In this case, many modification methods, such as chitosan modification (Cui et al., 2016), inorganic-organic modification (Ma et al., 2016) and metal modification (Cui et al., 2020) have been used to improve the adsorption capacities. A study showed that a 20% Al-modified biochar had a maximum PO₄³⁻ adsorption capacity of 57.49 mg/g, which was significantly higher than that of the unmodified biochar (Yin et al., 2018). Recent studies showed that Pisha sandstone exhibits effective retention of water (Jia et al., 2019) and adsorption of ammonium nitrogen (She et al., 2015b), but its phosphorus adsorption is poor (She et al., 2015b). The theoretical Langmuir phosphorus adsorption capacity of Pisha sandstone is 99 mg/kg (She et al., 2015a), which is still relatively low. Therefore, the phosphorus adsorption capacity of Pisha sandstone must be further enhanced to improve its effectiveness as a eutrophication treatment agent and broaden its utilization as a resource. Al modification is a possible means for improving phosphorus adsorption on Pisha sandstone and increasing the phosphorus content of soil without introducing new pollutants.

To date, few studies have investigated phosphorus adsorption by modified Pisha sandstone so far. Therefore, the objective of this study is to determine whether Al modification can enhance the phosphorus adsorption capacity of Pisha sandstone. The most economically appropriate modification ratio was determined, and a series of laboratory experiments were conducted to determine the adsorption mechanism and characteristics. This study can provide a theoretical basis for the resource utilization of Pisha sandstone as a eutrophication treatment agent and inspire new ideas for the application of other montmorillonite-enriched materials used in sandy soil amendments.