## 2 Materials, Experimental Procedures and Methods
### Analytical Methods
#### 2.3.2 Solid-Phase Characterization  
The mineralogy of GP and all Cu-GP samples was determined by X-ray diffraction (XRD) using a PANalytical X'Pert Pro operated at 40 kV and 40 mA (Co-K\(\alpha\)) and outfitted with a Scientific X'Celerator detector. The powdered specimens were prepared using the top loading technique and examined in the 4-85\({}^{\circ}\) 20 range using a step size of 0.008\({}^{\circ}\) 20 and 40 s count time per step. Further, the crystallinity of the Cu-bearing co-precipitates (see below) was determined by means of the full width at half-maximum (FWHM in \({}^{\circ}\) 20) values obtained from the respective peak of each phase with the highest diffracted intensity. The PANanalytical X'Pert Highscore Plus software and a pdf-4 database were used for the interpretation of the XRD patterns [43].  
Fourier transform infrared (FTIR) spectroscopy of GP was carried out in Attenuated Total Reflectance (ATR) mode using a PerkinElmer Frontier FTIR. Mid-infrared (MIR) spectra were collected in the 650-4000 cm\({}^{-1}\) range at a resolution of 2 cm\({}^{-1}\)[44]. Data processing was made via the Spekwin32 software (version 1.71.6.1).  
The major elemental composition of GP and of some Cu-GPs was analyzed with a PANalytical PW2404 wavelength dispersive X-ray fluorescence (XRF) spectrometer. Glass tablets were prepared in a PANalytical Perl'X 3 bead preparation system by the fusion of 0.5 g material (pre-dried at 110 \({}^{\circ}\)C) with 6.0 g lithium tetraborate (Li\({}_{2}\)B\({}_{4}\)O\({}_{7}\), Malvern Panalytical, Malvern, UK) at 1200 \({}^{\circ}\)C. The loss on ignition (LOI) was determined by gravimetric analysis of material residues glowed at 1050 \({}^{\circ}\)C for 1 h. The analytical error is \(<\)0.5 wt.% for the major elements, as determined by replicate analyses of USGS standards [43]. Comparison of Cu contents of Cu-GPs determined by ICP-OES and XRF analyses yielded a positive linear correlation with a slope of 0.98, an intercept of 0.1 and a R\({}^{2}\) value of 0.95 (\(n=9\)), so that only the ICP-OES data are presented in the following.  
Secondary electron (SE) images were acquired on a ZEISS DSM 982 Gemini scanning electron microscope (SEM) (ZEISS, Jena, Germany) operated at an accelerating voltage of 3-5 kV for microstructural characterization of the GP and Cu-GP samples. Representative material was prepared on SEM stubs, fixed with double-sided carbon (C) tape and subsequently C-coated to reduce charging [45]. Energy-dispersive X-ray spectroscopy (SEM-EDX) data were acquired on selected single spots using 15 kV accelerating voltage and subsequently used for mineral identification and quantification of the Cu content of the GP matrices. Chemical data (\(n=5/\)sample; average values and two standard deviations (2 SD) are reported) were obtained from element k-factors determined on mineral standards at an analytical precision of \(<\)1 at.% for K, Na, Al, Si and Cu analyses [26].  
The specific surface area (SSA) of the GP was measured before and after the batch experiments by the multi-point adsorption Brunauer, Emmett and Teller (BET) method using a Micromeritics FlowSorb II2300 (Micromeritics, Norcross, GA, USA) and a He(69.8)-N\({}_{2}\)(30.2) mixture as the carrier gas (analytical error: \(\pm\)10%).