Leon Lorenzen

Bio: Leon Lorenzen is an academic researcher from Stellenbosch University. The author has contributed to research in topics: Leaching (metallurgy) & Adsorption. The author has an hindex of 24, co-authored 59 publications receiving 2359 citations.

The potential use of geopolymeric materials to immobilise toxic metals: Part I. Theory and applications☆

Abstract: During the last decade geopolymerisation has emerged as a possible technological solution for the effective stabilisation and immobilisation of toxic materials. Despite the fact that this technology is based on a very old principle, surprisingly little is known about the nature of these reactions or their products. It is only in the last fifteen years that it has been rediscovered and attention has been drawn to its useful chemical and physical properties. This paper will therefore attempt to briefly discuss the available literature on geopolymerisation in terms of its history, reaction kinetics and structure as well as investigations into the application of geopolymerisation to various waste forms. It is evident from the literature that factors governing the formation of geopolymers are still poorly understood, although the physical and chemical properties suggest that these matrices are well suited for the immobilisation of toxic materials and specifically toxic metals. It is finally concluded that geopolymers offer attractive options towards simple industrial applications where large volumes of waste materials need to be stabilised. It must also be acknowledged that these advantages can only be applied optimally once all relevant interactions regarding the formation of geopolymers from waste materials have659 been quantified scientifically. Hence, further research is required regarding the formation of geopolymers and their application in industry.

Factors affecting the immobilization of metals in geopolymerized flyash

Abstract: Geopolymerization, a fairly new technology based on a very old principle, has emerged during the last few years as a possible solution to some waste stabilization and solidification problems. Some commercial successes have been achieved, although the technique remains fairly unknown as well as seemingly unpopular. It has been shown that most waste materials containing sources of silica and alumina should be capable of taking part in a geopolymerization reaction. In this article, flyash was used as a reactant in creating a geopolymeric matrix for the immobilization of process water containing 25,000 ppm of Cu or Pb cations. By means of X-ray diffraction, scanning electron microscopy (SEM), infrared spectroscopy, Brunauer-Emmett-Teller (BET), compressive strength, as well as kinetic leaching analyses, the main factors influencing matrix stability, immobilization efficiency, and therefore leaching behavior were investigated and discussed qualitatively. It was found that relatively high strengths could be obtained using low Ca flyash. The environment and coordination number of source aluminum and silica seemed to play a major role in the eventual matrix stability. Other factors influencing matrix stability include the alkali metal cation used as well as the type of metal being immobilized. The kinetics of leaching of immobilized metals from the geopolymerized flyash were qualitatively found to proceed along a combination of pore diffusion and boundary diffusion control mechanisms. It is finally concluded that immobilization of metals in geopolymerized flyash proceeds by a combination of physical encapsulation and chemical bonding, with adsorption also thought to play a role.

Factors affecting the mechanism of the adsorption of arsenic species on activated carbon

Abstract: The development of carbon-in-pulp (CIP) and resin-in-pulp (RIP) processes sparked off intensive investigations into the kinetics and mechanism of adsorption of various species onto activated carbon. The increased processing of complex sulphide ores (e.g. the bacterial oxidation of gold-bearing arsenopyrite) has led to higher levels of impurities such as arsenic in process streams. The removal of such impurities could become a new field of application for activated carbon. It was found that As(V) is more effectively removed from solution by using activated carbon with a high ash content. Pretreatment of the carbon with Cu(II) solutions improves its arsenic removal capacity. The optimum pH for arsenic adsorption by pretreated carbon is approximately 6. There are two mechanisms of arsenic adsorption which occur simultaneously. The arsenic in solution can form insoluble metal arsenates with the copper impregnated in the carbon. Arsenic is also adsorbed independently of the impregnated copper. Ion pair formation has been ruled out in the latter case. Arsenic desorption is easily achieved using strong acidic or alkaline solutions. When acidic solutions are used, copper is also eluted.

Mineralogy and uranium leaching response of low grade South African ores

Abstract: The efficiency of uranium leaching determines the economic viability of treating low grade uranium deposits, and is quite sensitive to ore characteristics. The interrelationship between mineralogy, mineral liberation and the leaching behaviour of uranium is not well defined. Uraninite’s leaching kinetics are well studied, but relatively little leaching research has been conducted for other uranium minerals. Dissolutions higher than 90% are very difficult to achieve under the normal operating conditions employed for acid leaching of South African ores. In this paper, a mineralogy-leachability explanation is presented to rationalise the difficulty in exceeding 90% dissolution from low grade uranium ores in the Vaal River region (averaging 0.3 kg U 3 O 8 /t). Based on the findings, further discussed in the paper, it appears that brannerite’s intrinsic inertness is responsible for not obtaining optimum recoveries. It is also shown that for low grade uranium ores in the Witwatersrand context, the real value of uranium leaching could lie in the unlocking of extra gold (0.4–0.6 g/t) by the uranium leaching process that typically precedes cyanide leaching of gold.

Ex situ diesel contaminated soil washing with mechanical methods

Abstract: Soil washing is a dynamic, physical process that cleans contaminated soil through transfer of the contaminant into a liquid stream. Three mechanical washing methods, i.e. jet reactor, attrition and ultrasonic washing were evaluated on a laboratory scale. The operating parameters for each washing process were investigated in detail and mechanisms are proposed for the observed effects. In a techno—economic comparison of the three methods, the jet reactor performed best, followed by ultrasonic and attrition washing. Experimental results suggested that a combination of the three washing techniques could yield residual diesel levels of less than 1000 mg/l for sand particles with an approximate size of 0.1 mm and an initial diesel content of 5%.

Arsenic removal from water/wastewater using adsorbents—A critical review

Abstract: Arsenic's history in science, medicine and technology has been overshadowed by its notoriety as a poison in homicides. Arsenic is viewed as being synonymous with toxicity. Dangerous arsenic concentrations in natural waters is now a worldwide problem and often referred to as a 20th-21st century calamity. High arsenic concentrations have been reported recently from the USA, China, Chile, Bangladesh, Taiwan, Mexico, Argentina, Poland, Canada, Hungary, Japan and India. Among 21 countries in different parts of the world affected by groundwater arsenic contamination, the largest population at risk is in Bangladesh followed by West Bengal in India. Existing overviews of arsenic removal include technologies that have traditionally been used (oxidation, precipitation/coagulation/membrane separation) with far less attention paid to adsorption. No previous review is available where readers can get an overview of the sorption capacities of both available and developed sorbents used for arsenic remediation together with the traditional remediation methods. We have incorporated most of the valuable available literature on arsenic remediation by adsorption ( approximately 600 references). Existing purification methods for drinking water; wastewater; industrial effluents, and technological solutions for arsenic have been listed. Arsenic sorption by commercially available carbons and other low-cost adsorbents are surveyed and critically reviewed and their sorption efficiencies are compared. Arsenic adsorption behavior in presence of other impurities has been discussed. Some commercially available adsorbents are also surveyed. An extensive table summarizes the sorption capacities of various adsorbents. Some low-cost adsorbents are superior including treated slags, carbons developed from agricultural waste (char carbons and coconut husk carbons), biosorbents (immobilized biomass, orange juice residue), goethite and some commercial adsorbents, which include resins, gels, silica, treated silica tested for arsenic removal come out to be superior. Immobilized biomass adsorbents offered outstanding performances. Desorption of arsenic followed by regeneration of sorbents has been discussed. Strong acids and bases seem to be the best desorbing agents to produce arsenic concentrates. Arsenic concentrate treatment and disposal obtained is briefly addressed. This issue is very important but much less discussed.

The Role of Inorganic Polymer Technology in the Development of ‘Green Concrete’

Abstract: The potential position of and drivers for inorganic polymers (“geopolymers”) as an element of the push for a sustainable concrete industry are discussed. These materials are alkali-activated aluminosilicates, with a much smaller CO 2 footprint than traditional Portland cements, and display very good strength and chemical resistance properties as well as a variety of other potentially valuable characteristics. It is widely known that the widespread uptake of geopolymer technology is hindered by a number of factors, in particular issues to do with a lack of long-term (20+ years) durability data in this relatively young research field. There are also difficulties in compliance with some regulatory standards in Europe and North America, specifically those defining minimum clinker content levels or chemical compositions in cements. Work on resolving these issues is ongoing, with accelerated durability testing showing highly promising results with regard to salt scaling and freeze–thaw cycling. Geopolymer concrete compliance with performance-based standards is comparable to that of most other high-strength concretes. Issues to do with the distinction between geopolymers synthesised for cement replacement applications and those tailored for niche ceramic applications are also discussed. Particular attention is paid to the role of free alkali and silicate in poorly-formulated systems and its deleterious effects on concrete performance, which necessitates a more complete understanding of the chemistry of geopolymerisation for the technology to be successfully applied. The relationship between CO 2 footprint and composition in comparison with Portland-based cements is quantified.

The geopolymerisation of alumino-silicate minerals

Abstract: Geopolymers are similar to zeolites in chemical composition, but they reveal an amorphous microstructure. They form by the co-polymerisation of individual alumino and silicate species, which originate from the dissolution of silicon and aluminium containing source materials at a high pH in the presence of soluble alkali metal silicates. It has been shown before that geopolymerisation can transform a wide range of waste alumino-silicate materials into building and mining materials with excellent chemical and physical properties, such as fire and acid resistance. The geopolymerisation of 15 natural Al–Si minerals has been investigated in this paper with the aim to determine the effect of mineral properties on the compressive strength of the synthesised geopolymer. All these Al–Si minerals are to some degree soluble in concentrated alkaline solution, with in general a higher extent of dissolution in NaOH than in KOH medium. Statistical analysis revealed that framework silicates show a higher extent of dissolution in alkaline solution than the chain, sheet and ring structures. In general, minerals with a higher extent of dissolution demonstrate better compressive strength after geopolymerisation. The use of KOH instead of NaOH favours the geopolymerisation in the case of all 15 minerals. Stilbite, when conditioned in KOH solution, gives the geopolymer with the highest compressive strength (i.e., 18 MPa). It is proposed that the mechanism of mineral dissolution as well as the mechanism of geopolymerisation can be explained by ion-pair theory. This study shows that a wide range of natural Al–Si minerals could serve as potential source materials for the synthesis of geopolymers.

Mechanism of geopolymerization and factors influencing its development: a review

Abstract: Geopolymerization is a developing field of research for utilizing solid waste and by-products. It provides a mature and cost-effective solution to many problems where hazardous residue has to be treated and stored under critical environmental conditions. Geopolymer involves the silicates and aluminates of by-products to undergo process of geopolymerization. It is environmentally friendly and need moderate energy to produce. This review presents the work carried out on the chemical reaction, the source materials, and the factor affecting geopolymerization. Literature demonstrates that certain mix compositions and reaction conditions such as Al2O3/SiO2, alkali concentration, curing temperature with curing time, water/solid ratio and pH significantly influences the formation and properties of a geopolymer. It is utilized to manufacture precast structures and non-structural elements, concrete pavements, concrete products and immobilization of toxic metal bearing waste that are resistant to heat and aggressive environment. Geopolymers gain 70% of the final strength in first 3–4 h of curing.