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Permeable reactive barriers (PRBs) are passive, in-situ remediation systems that comprise the filtration of a contaminated groundwater through a reactive media able to degrade, adsorb or precipitate the pollutants. Selection of the reactive media depends on the nature of the contamination. While the majority of PRBs are used to treat groundwater contaminated with chlorinated volatile organic compounds or acid mine drainage, the present study is focused on heavy metals, namely Fe, Al, Cu, Ni, Zn, Mn and As. To remove such inorganic pollutants, four methodologies are usually implemented: a) redox reaction, b) adsorption and ion exchange, c) biological treatment (e.g. metal sulphides precipitation) and d) pH control and precipitation. The present paper deals with the last methodology in which an adjustment of pH by alkalinity-generating materials leads to the precipitation of metal hydroxides and the decrease in the concentration of dissolved species. As the solubility of metals depends on their valence, redox-potential, concentrations and kinetics of reaction, the target pH varies with respect to the considered metal. A pH of about 10 allows the precipitation of hydroxides of divalent ions (zinc, manganese, copper, lead, nickel, cobalt and cadmium), while a pH of around 6-7 is adapted to trivalent ions (iron, aluminium and chromium). However, the accumulation of precipitates in the pore spaces can cause clogging of the pores, hence decrease the porosity and hydraulic conductivity of the reactive media and alter its efficiency. To validate this assumption, the neutralizing ability of alkaline materials such as activated zeolite and magnesite were examined in a test column by monitoring the resultant pH with attention to impact of precipitates on the hydraulic properties of reactive materials. Based on the test results, both materialls are considered suitable to remove the groundwater contaminants with high efficiency (up to 99%) and suitable grain size to encourage flow through the PRB.

This paper examines the effect of the nature of the precipitating agent (HCl, CO 2 and (NH 4 ) 2 CO 3 ) on the surface area of the resulting silica powders synthesized from serpentinite after its leaching in hydrochloric acid. In order to prepare a product consisting of small discrete particles polymerized into three-dimensional networks without using additives (surfactant and/or flocculant), synthesis from nearly-saturated sodium metasilicate solution at laboratory temperature under alkaline and acid-set conditions was applied in the present work. In highly acidic solution (e.g. at significant stochiometric excess of hydrochloric acid over the silicate) a regular density xerogel with a high specific surface area (541 m 2 g − 1 ) was achieved. Through synthesis of silica gel by acidification using CO 2 and (NH 4 ) 2 CO 3 , the values 392 m 2 g − 1 and 414 m 2 g − 1 respectively of the specific surface area were obtained. The microporosity of silica samples precipitated by HCl was confirmed by t -methods and α -methods, whereas powders prepared using CO 2 and (NH 4 ) 2 CO 3 were identified as being mesoporous. The mesopore size distribution calculated using the BJH method and DFT model shifted with rising pore diameter in the following order: silica precipitated by HCl, (NH 4 ) 2 CO 3 and CO 2 .

The overall process for producing pure magnesium compounds from serpentinite usually starts with acid leaching. However, if serpentinite is calcined prior to leaching, not only faster magnesium dissolution is achieved, but use of thermally activated serpentine also significantly reduces the problems relating to corrosion of the leaching equipment, because less aggressive leaching agents can be used and/or lower leaching temperatures and pressures can be applied. This paper compares how calcination influences the dissolution behaviour of magnesium in solutions of hydrochloric acid, acetic acid and ammonium chloride. Fine-grained serpentinite, characterized by the extent of serpentine decomposition between 85 and 95%, displayed the highest reactivity. The initial magnesium dissolution rate of calcined serpentinite was up to 30-, 125- and 165-times higher as compared to that of uncalcined serpentinite in solutions of hydrochloric acid, acetic acid and ammonium chloride, respectively, under identical reaction conditions.

The kinetics of the reaction between particulate stibnite and mixed Na2S + NaOH solutions were studied. The effects of the concentration of the Na2S and NaOH (from 0.5% to 2.0%), temperature (from 292 K to 327 K) and particle size (from 40 μm to 500 μm) were investigated. The results are presented in terms of the shrinking (non-reacted) core model and the shrinking porous-particle model. The apparent activation energy was approximately 44 kJ mol− 1, and the apparent reaction order for Na2S varied from 1.4 to 1.7. The calculated values of the kinetic parameters indicate that the leaching process is controlled by both the intrinsic chemical reaction between Sb2S3 and Na2S at the liquid/solid interface and pore diffusion.

The kinetics of the dissolution of particulate sintered polycrystalline MgO under advanced kinetic regime conditions were investigated, with special regard to the effects of concentration of H+ ions, temperature and the type of the inorganic acid used (HCl, HNO3 and H2SO4). It was observed that the rate of chemical dissolution of MgO accelerated with increase in both concentration of H+ ions (from 10−4 to 10−2 M) and temperature (from 25 to 60 °C). The overall process of MgO dissolution was found to be controlled by the surface chemical reaction of MgO with H+ ions. The presented values of the reaction order for H+ ions are consistent, allowing for experimental error, with current kinetic theories and most other experiments. However, the rate-controlling step could not be assigned unambiguously, because the same value of the reaction order for H+ ions can be predicted by various theories for different rate-determining steps. Measured values of the reaction order for H+ ions are indicative that the rate is limited by the second protonation reaction, i.e. the slow reaction of surface hydroxide with a second proton. Though the reaction order for H+ ions seems not to be affected by the type of acid used, the apparent activation energy changes in the order HCl ≤ HNO3

Kinetics of the dissolution of particulate polycrystalline MgO in excess of hydrochloric acid were investigated, with special regard to the effect of activity of H + ions, temperature and particle size. The results are presented in terms of the shrinking particle model. The overall process is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface. The apparent activation energy was 58 kJ mol −1 . Acid concentration (from 0.5 M to 5.1 M) was surprisingly found to have a decelerating effect on the dissolution rate of MgO, resulting in negative values of reaction order for H + ions (from −0.16 to −0.18). We conclude that the regime studied here, characterised by decrease in the dissolution rate of MgO with an increase in H + activity, occurs under far from mass-transfer control conditions in the solutions with low Mg 2+ :H + molar ratio.

Kinetics of the reaction between dead-burned magnesite and hydrochloric acid were investigated with special regard to the rate of chemical dissolution of MgO. The effect of process parameters viz. temperature, activity of H + ions, and particle size and composition of the solid was investigated. The dissolution of MgO was strongly affected by temperature (from 45 to 75 °C) and particle size (from 63 to 355 μm), while the effect of composition of the solid was weak. The results are presented in terms of the shrinking particle model. The dissolution of MgO is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface. The apparent activation energy was 58–64 kJ mol −1 . Acid concentration (from 1.0 to 5.3 M) was surprisingly found to have a decelerating effect on the dissolution rate of MgO, resulting in negative values of reaction order for H + ions (from −0.1 to −0.2). We conclude that the leaching behaviour studied here corresponds to the regime referred to in the literature as the “initial stage of dissolution of MgO”. This regime, during which the dissolution rate increases rapidly despite a decrease in H + concentration, is characterised by far from mass-transfer control conditions, which was exactly the situation analysed in the present work.

The leaching of calcined magnesite using ammonium chloride has been tested in a pH-stat to ascertain the effect of process parameters viz. temperature, concentration of NH 4 Cl, pH, and particle size and reactivity of the solid. The main purpose of the approach adopted was to keep the lixiviant composition constant during individual measurements. A simple mathematical model has been used to describe the dissolution of magnesium during leaching and to analyse the kinetic data. The apparent activation energy of leaching was found to depend on the particle size of the solid — the values 57.8 and 48.5 kJ mol −1 were obtained for the leaching of particles of −100+90 and −180+160 μm, respectively. It was concluded that the leaching process is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface and by pore diffusion.