Challenging the Positive Role of Calcium Ions in Pyrite Flocculation: Evidence of Adverse Effects from Acrylamide Flocculant Adsorption Studies and Molecular Simulations

A Mabudi and R Ahmadi, ARABIAN JOURNAL FOR SCIENCE AND ENGINEERING, 50, 20803-20823 (2025).

DOI: 10.1007/s13369-025-10343-2

This study re-examines the role of calcium ions (Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument) in pyrite flocculation using acrylamide-based flocculants (A26 and A27), combining experimental and molecular dynamics (MD) approaches. Contrary to conventional wisdom, results demonstrate that Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument adversely influences flocculation efficiency. Laboratory tests showed that increasing Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument concentrations (up to 150 mg/L) reduced settling velocities by up to 50% and increased turbidity, with the high-acrylamide flocculant A27 being particularly affected. MD simulations revealed that Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument neutralizes negative charges on both pyrite surfaces and flocculant polymers, weakening critical hydrogen bonding and electrostatic interactions. This disruption caused a 2 angstrom shift in flocculant adsorption position and decreased floc density by 15-20%, leading to less stable aggregates. Performance depended strongly on flocculant composition: A27 (17:1 acrylamide: acrylic acid ratio) outperformed A26 (9:1 ratio) due to enhanced hydrogen bonding, but both suffered efficiency losses with Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument. Optimal flocculation occurred at pH 10.5 without Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument, where A27 achieved 142.07 m/h settling velocity. FTIR analysis confirmed electrostatic interactions dominated the adsorption mechanism, with no evidence of Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument bridging. These findings challenge established paradigms about Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument's beneficial role and provide molecular-level insights for optimizing flocculant design in mineral processing, particularly for Ca2+\documentclass12ptminimal \usepackageamsmath \usepackagewasysym \usepackageamsfonts \usepackageamssymb \usepackageamsbsy \usepackagemathrsfs \usepackageupgreek \setlength\oddsidemargin-69pt \begindocument$$Ca<^>2+$$\enddocument-rich systems. The study highlights the need to reconsider water treatment strategies in mining operations where calcium concentrations may compromise flocculation performance.

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