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Salt Lakes of the African Rift System: a Valuable Research Opportunity for Insight Into Nature's Concenrtated Multi-Electrolyt

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Salt Lakes of the African Rift System: a Valuable Research Opportunity for Insight Into Nature's Concenrtated Multi-Electrolyt Tanz. J. Sci. 38(3), 2012 SALT LAKES OF THE AFRICAN RIFT SYSTEM: A VALUABLE RESEARCH OPPORTUNITY FOR INSIGHT INTO NATURE’S CONCENRTATED MULTI-ELECTROLYTE SCIENCE JYN Philip and DMS Mosha Chemistry Department, University of Dar es Salaam, P.O. Box 35061 Dar es Salaam, Tanzania ABSTRACT The Tanzanian rift system salt lakes present significant cultural, ecological, recreational and economical values. Beyond the wealth of minerals, resources of these rift salt lakes include unique wildlife, for instance Lesser Flamingo ( Phoenicopterus minor ) and Cichlids ( Alcolapia spp ), which adapted chemically aggressive environments like that of Lake Natron. The general hydrochemical + + 2+ 2+ 2- - - equilibria of these salt lakes is governed by Na > K >> Ca ~ Mg : (CO 3 + HCO 3 ) > Cl > 2- - SO 4 ~ F ionic concentration pattern. Salt accumulation mechanism in these endorheic lakes is considered to base on thermo-mechanical disintegrative stress of the geochemistry sourcing. The rains chemically interact with rift terrains, which are dominated with natrocarbonatitic volcanic materials, thus transporting dissolved salts to the lakes. In addition, hot springs around the shores of these lakes carry significant salinity loads into the lakes. The salt reserves magnifies through recurring evaporative concentration in seasonal cycles. Despite this huge wealth of mineral resource potential of the Tanzanian rift salt lakes, they are still underutilized, thus contributing less into the national economy. It is envisaged that studies on phase rule on separation of pure salts from lake brines as demonstrated herein have a direct industrial value addition to Tanzanian rift salt lakes. INTRODUCTION electrolyte resource into the Tanzanian Economically significant reserves of the economy. This is indeed in conflict with the multi-electrolyte concentrates/evaporites pressing country-wide demand for soda ash that typify the lakes of the African rift and allied minerals as key industrial system are to be found in Tanzania in host feedstock, which currently wholly relies on rift lake locations fitting the description importation. This realization has prompted a “endorheic” (closed) basins. Most prominent re-launch of local investigative chemical among these are lakes Natron, Manyara and scientific effort by local research groups to Eyasi plus smaller ones of lesser expanse provide quantity and quality evaluation and and prominence such as Kitangiri, updating, as well as harness the scholarship Balangida, Singidani, Balangidalelu, Basotu value availed by the challenging domain of and Basoda. natural hyper-saline multi-electrolyte science. Whereas the chemistry and aquatic science of salt lakes globally elsewhere have Important scientific contrast and departures benefited from substantial scientific from dilute aqueous text book chemistry research; critical knowledge gaps, scanty have been shown to characterize and apply and outdated scientific literature and to hyper-concentrated multi-electrolyte diminished prioritization for the Tanzanian chemical environments. The chemistry of resources have been responsible for the extremely concentrated electrolytes is stagnation in research productivity. As a known to approach that of salt melts, a result there is meager input of this salt lake frontier scientific domain that demands the 1 Philip and Mosha - Salt Lakes of the African Rift System … development of newer models for a clear of the Tanzanian and regional rift valley salt understanding of the governing equilibria. lakes fall under the category of “ soda lakes ” Chemical speciation, the new equilibria and of high pH ≥ 10 (from anionic dominance of processes such as pH, p Ka ’s buffer carbonates). The Na + > K + >> Ca 2+ ~ Mg 2+ : 2- - - 2- - capacities, the extreme aggressive chemical (CO 3 + HCO 3 ) > Cl > SO 4 ~ F ionic environment and adaptive mechanisms concentration pattern is typical (Table 1) evolved by resident bioforms, the phase rule save for few cases such as Lake Singidani 2- - tool for extraction and separation science for where [CO3 + HCO 3 ] is minor and closely pure salts may be cited among interesting mirrors the non-soda Dead Sea brine rewarding key research themes thus chemistry. catalyzed and pursued in the project. Many Table 1 : Trend of brine ionic concentrations in Tanzanian rift valley lakes (Kameka 2006). Cations Anions 2+ + 2+ + - - 2- 2- Dead Sea Mg > Na > Ca > K Cl > Br >> SO 4 ~ CO 3 + + 2+ 2+ 2- - - 2- - L. Natron Na > K >> Ca ~ Mg (CO 3 + HCO 3 ) > Cl > SO 4 ~ F + + 2+ 2+ 2- - - - 3- Ethiopian Lakes Na > K > Ca ~ Mg (CO 3 + HCO 3 ) > Cl > F > PO 4 + + 2+ - 2- 2- - Australian Lakes Na > K > Ca Cl > SO 4 > (CO 3 + HCO 3 ) + + 2+ 2+ - 2- - 2- - L. Eyasi Na > K >> Ca ~ Mg Cl > (CO 3 + HCO 3 ) > SO 4 >F + 2+ 2+ + - 2- 2- - - L. Singidani Na > Ca ~ Mg > K Cl > SO 4 > (CO 3 + HCO 3 ) > F + + 2+ 2+ - 2- - 2- 3- L. Balangida Na > K >> Ca ~ Mg Cl > (CO 3 + HCO 3 ) > SO 4 > PO 4 + 2+ + 2+ - 2- - 2- - 3- L. Kitangiri Na > Ca > K > Mg Cl > (CO 3 + HCO 3 ) > SO 4 > F > PO 4 + + 2+ 2+ 2- - - 2- 3- - L. Manyara Na > K > Ca ~ Mg (CO 3 + HCO 3 ) > Cl > SO 4 > PO 4 > F + + 2+ 2+ 2- - - 2- - L. Magadi Na > K >>Ca ~ Mg (CO 3 + HCO 3 ) > Cl > SO 4 > F + 2+ 2+ + - 2- 2- - - - Sea Water Na >> Mg > Ca ~ K Cl >> SO 4 > (CO 3 + HCO 3 ) > Br > F Ionic concentrations are location and season individual salt lakes. These lakes are proving dependent, peaking downstream in open to host competitively ample resources stabilized lake brines and swinging with the locally, regionally and globally, and in some dry and wet seasons where they become of the instances such as Lake Natron, the concentrated during dry spells and vice tonnages excel any single source elsewhere versa during the wet season. The salt worldwide in terms of soda ash reserve accumulation mechanism hinges in salt lake volumes. basin dischargelessness of the endorheic basin which guarantees solute retention, RESOURCES OF THE RIFT SALT while the negative hydrological balance, LAKES high desiccating temperatures and aridity of East African Rift lakes (Fig. 1) and rift localities is responsible for hyped levels Tanzanian lakes in particular, support a of the evaporative processes to yield the flourishing tourism industry centred on a electrolyte concentrates. Selective salt unique wildlife endowment particularly the precipitation into bedded sediments is cross border migratory Lesser Flamingo another modifier of the chemical (Phoenicopterus minor ) populations and a constitution patterns. The mineral salt rich bird life associated with lake resource potential of local salt lakes as ecosystems. Apart from the resident fish sought out by our local effort as well as varieties, for example Cichlids such as literature sources is presented here in terms Alcolapia spp of lake Natron, that can adapt of tonnage estimates for the major to these extreme chemically aggressive economically significant salts for the 2 Tanz. J. Sci. 38(3), 2012 environments, fishing is not an economically significant industry on the soda lakes. Figure 1 : Closed lake basins of the East African Rift System. The mineral potential of Tanzanian salt Lake Manyara and the largest known such lakes remain largely unexploited to-date deposit in the Southern Africa Development except the Minjingu phosphate deposits (Orr Cooperation (SADC) region. and Grantham 1931, Qulwi 1995) which is an ancient dried-up lake bed extension of 3 Philip and Mosha - Salt Lakes of the African Rift System … Unlike Lake Magadi (Kenya) where mining of nature’s cheaper supplies. In the commercial exploitation commenced in African region, Lake Magadi continues to 1914, commercial salt production has dominate the production while Sua Pan remained virtually insignificant for the (Botswana) and Abiata (Ethiopia) are abundantly rich Tanzanian salt and soda significant producers. An estimate of the lakes, save for peripheral domestic breakdown of the reserves in Tanzanian salt utilization such as medicinal, snuff additive, lakes (Kameka 2006, Mwakoba et al. 1992), laundry soda and food cookability improver. in terms of tonnage, is presented in Table 2. Soda ash is the principal industrial feedstock Lake Natron is singularly impressive and chemical now wholly globally sourced from surpasses most others in the rift zone and in nature’s soda lake reserves as the cheaper the region beyond (globally), and the alternative, after the more expensive Solvay economic development of its key industrial and Le Blanc Na 2CO 3 industrial production mineral reserves promises a new era of process options were abandoned after meaningful resource prosperity. proving unattractive cost-wise compared to Table 2: The mineral potential of Tanzanian rift lakes in tonnage estimates (Kameka 2006). L. Natron L. Balangida L. Eyasi NaCl 20,000,00 240,000 2,471,000 2- - Na - (CO 3 + HCO 3 ) 26,000,000 208,000 2,118,000 Na 2SO 4 900,000 152,000 n.d. KCl n.d. 8700 n.d. Na 3PO 4 30,000 n.d. n.d. n.d = not determined Hydrochemical equilibria Mosha et al . 1996) and defy conventional − − For soda lakes, carbonates ( CO 2 + HCO ) dilute chemical methods of interpretation. 3 3 are the dominant brine electrolytes FACTORS AFFECTING THE concentration-wise in the majority of cases. COMPOSITION OF LAKE BRINES The governing brine chemical equilibria, pH such as pH, carbonate, fluoride, phosphate This is strictly a measure of a where a = and silicate, modified by high brine ionic H+ activity. It is known that a ≠ effective [H +] strength conditions, has been explored H+ if the activity coefficient ƒ ≠ unity, due to (Guest and Stevens 1951, Kameka 2006, restrictions on ion mobility and reactivity. In Mosha et al . 1996, Mwakoba et al . 1992, our work, we have adopted that a two pH Ngambeki 1996, Qulwi 1995). unit downward adjustment is required (Mwakoba et al ., 1992) for a direct The equilibria are fundamentally important conventional pH meter readout (which bases and have a governing determining role on on the manufacturer’s calibrants) to cater for solubility, formation, sedimentation, ion- this fundamental complication.
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