Unravelling the pH-driven multiscale cascade of hematite flocculation: From interfacial tuning to structural assembly and sedimentation dynamics

Abstract

Efficient flocculation and sedimentation of ultrafine hematite remain a key challenge in mineral processing. This study elucidates the pH-dependent flocculation behaviour of hematite with anionic polyacrylamide (APAM) using a multi-scale correlation framework integrating interfacial analysis, structural characterization, and sedimentation evaluation. Increasing pH induces progressive surface deprotonation, yielding a more negative hematite surface and enhanced APAM adsorption from 0.106 to 0.186 mg/m². FTIR, XPS, and molecular dynamics simulations consistently reveal strengthened Fe–OOC coordination, intensified hydrogen bonding, and more stabilised polymer conformations under alkaline conditions. Microscopy, SEM, and FBRM show that alkaline conditions facilitate the formation of larger and denser flocs, with size increasing from 56 to 982 μm and fractal dimension from 1.44 to 1.87. These structural changes markedly improve sedimentation performance, reducing turbidity from 436.8 to 76.7 NTU and increasing settled solids from 35.94 to 52.43 percent. The proposed multi-scale correlation model quantitatively links interfacial chemistry, floc structural evolution, and settling behaviour, providing a unified mechanistic basis for pH-regulated hematite flocculation. This framework not only advances understanding of polymer–mineral interactions but also offers practical guidance for optimising solid–liquid separation and tailings-water recycling in fine mineral beneficiation.

Keywords

Ultrafine hematite / Anionic polyacrylamide / pH regulation / Multi-scale flocculation mechanism / Floc structure evolution / Solid–liquid separation

CRediT authorship contribution statement

Zhangke Kang: Writing – original draft, Data curation. Yanbai Shen: Writing – review & editing. Baoyu Cui: Funding acquisition. Shuling Gao: Resources. Wengang Liu: Methodology. Qiang Zhao: Formal analysis.

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.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (Nos. 51974066 and 52204265), and the Fundamental Research Funds for the Central Universities (No. N2201004).

Supplementary data

Supplementary data to this article can befound online at https://doi.org/10.1016/j.ijmst.2026.02.003.

References

[1] Kang ZK, Shen YB, Cui BY, Zhao Q, Lv HY, Wu ML, et al. Mechanism and dynamic regulation of synergistic flocculation in multi—sized ultrafine hematite: Experimental characterization and kinetic modeling. Water Res 2025; 285:124066.

[2] Niu FS, Chen YY, Zhang JX, Liu F, Wang ZY. Selective flocculation—flotation of ultrafine hematite from clay minerals under asynchronous flocculation regulation. Int J Min Sci Technol 2024; 34(11):1563-74.

[3] Long HL, Zhu DQ, Pan J, Li SW, Yang CC, Guo ZQ, et al. A critical review on metallurgical recovery of iron from iron ore tailings. J Environ Chem Eng 2024; 12(2):112140.

[4] Zhu XL, Jiang LL, Zhang SX, Yang JY, Liang JJ, Zheng HL, et al. Evaluation an anionic polyacrylamide flocculant with microblock structure in the hematite wastewater treatment: Characterization and flocculation performance. Colloids Surf A Physicochem Eng Aspects 2024; 693:134072.

[5] Zhang JL, Guo YF, Zheng FQ, Wang S, Chen F, Yang LZ. High efficiency filtration and optimization of pelletizing performance of fine hematite concentrate using sulfuric acid filter aid: Behavior and mechanism. Powder Technol 2024; 448:120260.

[6] Silva RCF, Ardisson JD, Cotta AAC, Araujo MH, de Carvalho Teixeira AP. Use of iron mining tailings from dams for carbon nanotubes synthesis in fluidized bed for 17a—ethinylestradiol removal. Environ Pollut 2020; 260:114099.

[7] Wang ZJ, Wu HQ, Xu YB, Shu KQ, Fang S, Xu LH. The effect of dissolved calcite species on the flotation of bastnaesite using sodium oleate. Miner Eng 2020; 145:106095.

[8] Xie Y, Yin WZ, Yao J, Xue FJ, Liu JY, Ban XQ. Application and mechanism of an efficient hydrophobic flocculant CPAM in the flocculation reverse flotation of hematite and microfine—grained chlorite. J Environ Chem Eng 2024; 12(3):112608.

[9] Zhao SK, Xiao H, Chen YS, Qi YZ, Yan CH, Ma RX, et al. Photocatalytic degradation of xanthates under visible light using heterogeneous CuO/TiO₂/montmorillonite composites. Green Smart Min Eng 2024;1(1):67-75.

[10] Asgari K, Khoshdast H, Nakhaei F, Garmsiri MR, Huang QQ, Hassanzadeh A. A review on floc—flotation of fine particles: technological aspects, mechanisms, and future perspectives. Miner Process Extr Metall Rev 2024; 45(7):669-96.

[11] Alvim ES, Fernandes Lima RM. Selective flocculation/magnetic separation of ultrafine iron ore particles with corn starch and polyacrylamides: A comparative study. Miner Process Extr Metall Rev 2024; 45(6):606—15.

[12] Jiang XY, Shen YB, Lv HY, Chen K, Xia Y, Zhao SK, et al. Highly improved degradation of benzohydroxamic acid by bubble—propelled heterogeneous Fenton catalysts of halloysite/MnFe₂O₄: Enhancing mass transfer and accelerating Fe/Mn cycle. Appl Catal B Environ Energy 2025; 362:124726.

[13] Lai HJ, Fang HW, Huang L, He GJ, Reible D. A review on sediment bioflocculation: dynamics, influencing factors and modeling. Sci Total Environ 2018; 642:1184-200.

[14] Karoyo AH, Wilson LD. A review on the design and hydration properties of natural polymer—based hydrogels. Materials 2021; 14(5):1095.

[15] Laura Pinedo M, Riascos BD, Quintero XE, Costa C. Mechanism of pH sensitive flocculation for organic load and colour reduction in landfill leachate. Waste Manag 2022; 144:163-72.

[16] Arinaitwe E, Pawlik M. A role of flocculant chain flexibility in flocculation of fine quartz. Part I. Intrinsic viscosities of polyacrylamide—based flocculants. Int J Miner Process 2013; 124:50-7.

[17] Kallay N, Dojnovic Z, Cop A. Surface potential at the hematite—water interface. J Colloid Interface Sci 2005; 286(2):610-4.

[18] Godymchuk A, Papina I, Karepina E, Kuznetsov D, Lapin I, Svetlichnyi V. Agglomeration of iron oxide nanoparticles: pH effect is stronger than amino acid acidity. J Nanopart Res 2019; 21(10):208.

[19] Engwayu J, Pawlik M. Adsorption of anionic polymers on hematite—a study of zeta potential distributions. Miner Eng 2020; 148:106225.

[20] Dong HY, Gao BY, Yue QY, Wang Y, Li Q. Effect of pH on floc properties and membrane fouling in coagulation—ultrafiltration process with ferric chloride and polyferric chloride. Chemosphere 2015; 130:90—7.

[21] Li SL, Gao LH, Cao YJ, Gui XH, Li Z. Effect of pH on the flocculation behaviors of Kaolin using a pH—sensitive copolymer. Water Sci Technol 2016; 74(3):729—37.

[22] ATANGANA E. Development of Modified Biopolymer Adsorbents From Natural Polysaccharides For Renewal Of Abattoir Wastewater[D]; Bloemfontein:Central University of Technology, Free State, 2019.

[23] Yue T, Wu XQ, Chen X, Liu TY. A study on the flocculation and sedimentation of iron tailings slurry based on the regulating behavior of Fe³⁺. Minerals 2018; 8(10):421.

[24] Spencer KL, Wheatland JAT, Bushby AJ, Carr SJ, Droppo IG, Manning AJ. A structure—function based approach to floc hierarchy and evidence for the non—fractal nature of natural sediment flocs. Sci Rep 2021; 11(1):14012.

[25] Han CC, Tan YY, Chu LS, Song WD, Yu X. Flocculation and settlement characteristics of ultrafine tailings and microscopic characteristics of flocs. Minerals 2022; 12(2):221.

[26] Nabweteme R, Yoo M, Kwon HS, Kim YJ, Hwang G, Lee CH, et al. Application of the extended DLVO approach to mechanistically study the algal flocculation. J Ind Eng Chem 2015; 30:289—94.

[27] Ward BJ. Settling and dewatering of fecal sludge: building fundamental knowledge for improved global sanitation. ETH Zurich 2022.

[28] MACIVER M R. Composition and structure in flocculated mineral systems; University of British Columbia, 2022.

[29] Kang X, Cao JN, Bate BT. Large—strain strength of polymer—modified kaolinite and fly ash—kaolinite mixtures. J Geotech Geoenviron Eng 2019; 145(2):04018106.

[30] Jiang T, Wu YK, Liu CL, Whittle AJ, Guo DD, Zhang GP. Kinetics of flocculated illite suspensions affected by ionic strength, pH, and hydrodynamic shearing. Appl Clay Sci 2022; 221:106462.

[31] Tran D, Strom K. Floc sizes and resuspension rates from fresh deposits: Influences of suspended sediment concentration, turbulence, and deposition time. Estuar Coast Shelf Sci 2019; 229:106397.

[32] Leiva WH, Toro N, Robles P, Quezada GR, Salazar I, Jeldres R. Clay tailings flocculated in seawater and industrial water: Analysis of aggregates, sedimentation, and supernatant quality. Polymers 2024; 16(10):1441.

[33] Guo B, Zhan XL, Jiang KX, Xie HZ, Deng RD. Flocculation behavior of ultrafine silica particles in acid leaching pulp by nonionic polymeric flocculants. Minerals 2023; 13(4):582.

[34] Mancell—Egala WASK, Su CY, Takacs I, Novak JT, Kinnear DJ, Murthy SN, et al. Settling regimen transitions quantify solid separation limitations through correlation with floc size and shape. Water Res 2017; 109:54-68.

[35] Neisiani AA, Saneie R, Mohammadzadeh A, Wonyen DG, Chelgani SC. Polysaccharides—based pyrite depressants for green flotation separation: An overview. Int J Min Sci Technol 2023; 33(10):1229—41.

[36] Adhikari NM, Zarzycki P, Wang ZM, Rosso KM. pH—dependent reactivity of water at MgO(100) and MgO(111) surfaces. PCCP 2025; 27(8):4343-54.

[37] Wang JW. Molecular—level insights into the adsorption of low—molecular—weight biomolecules at water—mineral interfaces. Evanston: Northwestern University, 2025.

[38] Quezada GR, Jeldres M, Toro N, Robles P, Toledo PG, Jeldres RI. Understanding the flocculation mechanism of quartz and kaolinite with polyacrylamide in seawater: a molecular dynamics approach. Colloids Surf A Physicochem Eng Aspects 2021; 608:125576.

[39] Liu FY, Zou WJ, Xiao T, Hu WT, Zhang M, Zhang ZJ, et al. Molecular dynamics simulations investigating the influence of different chitosan—based graft polymer monomer types on their flocculation behavior. Colloid Polym Sci 2025; 303(6):1071-80.

[40] Peng SN, Wang ZY, Yu PF, Liao GY, Liu R, Wang DS, et al. Aggregation and construction mechanisms of microbial extracellular polymeric substances with the presence of different multivalent cations: Molecular dynamic simulation and experimental verification. Water Res 2023; 232:119675.

[41] Quezada GR, Vargas AA, Nieto S, García KI, Robles P, Jeldres RI. Molecular dynamics study of polyacrylamide and polysaccharide—derived flocculants adsorption on Mg(OH)(2) surfaces at pH 11. Polymers 2025; 17(2):227 [PubMed].

[42] Lockwood APG, Wadsley G, Warren NJ, Peakall J, Webber GB, Wanless EJ, et al. Amphiphilic block copolymers as dual flocculation—flotation agents for rapid solid—liquid separation of radioactive wastes. Sep Purif Technol 2023; 323:124387.

[43] Kang ZK, Cui BY, Shen YB, Zhao Q, Du HY, Zhang YY. Ultrafine hematite flocculation: in—depth analysis of multi—factor and multi—index coordinated optimisation. Process Saf Environ Prot 2025; 198:107211.

[44] Scholz D, Stirner T. Convergence of surface energy calculations for various methods: (001) hematite as benchmark. J Phys Condens Matter 2019; 31(19):195901.

[45] Shrimali K, Jin JQ, Hassas BV, Wang XM, Miller JD. The surface state of hematite and its wetting characteristics. J Colloid Interface Sci 2016; 477:16-24.

[46] Jeldres RI, Fawell PD, Florio BJ. Population balance modelling to describe the particle aggregation process: A review. Powder Technol 2018; 326:190-207.