Speciation analysis and formation mechanism of iron-phosphorus compounds during chemical phosphorus removal process

Qian Ping; Bingqian Zhang; Zhipeng Zhang; Kexin Lu; Yongmei Li

doi:10.1016/j.chemosphere.2022.136852

Speciation analysis and formation mechanism of iron-phosphorus compounds during chemical phosphorus removal process

Chemosphere. 2023 Jan:310:136852. doi: 10.1016/j.chemosphere.2022.136852. Epub 2022 Oct 11.

Authors

Qian Ping¹, Bingqian Zhang², Zhipeng Zhang², Kexin Lu², Yongmei Li³

Affiliations

¹ State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, PR China; Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China. Electronic address: pingqian@tongji.edu.cn.
² State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, PR China.
³ State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, PR China; Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China. Electronic address: liyongmei@tongji.edu.cn.

PMID: 36241115
DOI: 10.1016/j.chemosphere.2022.136852

Abstract

Iron (Fe) salt was applied extensively to remove phosphorus (P) in wastewater treatment plants (WWTPs). Exploring the formation mechanism of iron-phosphorus compounds (FePs) during the chemical P removal (CPR) process is beneficial to P recovery. In this study, the performance of P removal, FePs speciation analysis and the kinetics of P removal under different conditions (pH, Fe/P molar ratio (Fe/P_mol), type of Fe salt, dissolved organic matters) were comprehensively investigated. More than 95% of P was removed under the optimal conditions with pH = 4.7, Fe/P_mol = 2, FeCl₃ or polymeric ferric sulfate (PFS) as the coagulant. The FePs formation mechanism was considerably influenced by reaction conditions. Iron-phosphate compounds were the dominant FePs species (>76%) at pH < 6.2, while more iron oxides were formed at pH ≥ 6.2 with decreased P removal efficiency. When the initial Fe/P_mol was 2, iron-phosphate compound was the only product that was formed by the reaction between PO₄^3- and Fe(III) or Fe(II) ions directly. More iron oxides were generated when the initial Fe/P_mol was 1 or 3. At Fe/P_mol = 1, the Fe(III) was hydrolyzed to form iron oxides and trapped PO₄^3-, while at Fe/P_mol = 3, iron-phosphate compounds were produced firstly and the remaining Fe(III) was hydrolyzed to form iron oxides. The pseudo-second-order kinetic model simulated the chemical P removal process well. The reaction rate of P with Fe(II) was slower than that with Fe(III), but complete removal was still achieved when the reaction time was more than 30 min. Poly-Fe salt exhibited a fast P removal rate, while the removal efficiency depended on its iron content. Organic matters in wastewater with large molecular weight and multiple functional groups (such as humic acids) inhibited P removal rate but hardly affect the removal amount. This study provides an insight into CPR by Fe salts and is beneficial for P recovery in WWTPs.

Keywords: Chemical P removal Process; Formation mechanism; Influencing factors; Iron-phosphorus compounds (FePs); Kinetics; Speciation.

MeSH terms

Ferric Compounds* / chemistry
Ferrous Compounds / chemistry
Iron / chemistry
Iron Compounds*
Oxides / chemistry
Phosphates
Phosphorus / chemistry

Substances

ferric oxide
Ferric Compounds
Phosphorus
Iron
Iron Compounds
Phosphates
Ferrous Compounds
Oxides