Surface characteristics and aggregation of microbiologically produced sulphur particles in relation to the process conditions

B SURFACES ELSEVIER Colloids and Surfaces B: Biointerfaces (1996) 115-129 Surface characteristics and aggregation of microbiologically produced sulphur particles in relation to the process conditions A Janssen aT*,A de Keizer b, A van Aelst ‘, R Fokkink b, H Yangling a, G Lettinga a a Department of Environmental Technology, Agricultural University, Wageningen, Bomenweg 2, 6703 HD Wageningen, The Netherlands b Department of Physical and Colloid Chemistry, Agricultural University, Wageningen, Dreijenplein 6, 6703 HD Wageningen, The Netherlands ’ Department of Plant Cytology and Morphology, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands Received 12 June 1995; accepted 26 September 1995 Abstract The effect of surface properties and the effects of several process conditions, e.g loading rate, ionic strength and the presence of polymers, on the degree of aggregation of sulphur particles were studied Sulphur is formed under oxygenlimiting circumstances during the partial oxidation of sulphide by a mixed culture of thiobacillus-like bacteria Since the freshly excreted particles are in a colloidal state, with a diameter of approximately 100 nm, their aggregation is a prerequisite in order to obtain a satisfactory sedimentation Titration experiments revealed that the negative sulphur surface charge is determined by the presence of multiple functional groups Attention was also paid to the effect of the chain length, hydrophilicity and charge of a number of dissolved polymers on the degree of sulphur aggregation The degree of polymer adsorption on the sulphur surface mainly depends on the hydrophobicity and charge of the polymer Since the charge of biologically produced sulphur is negative at pH 8.0, a highly charged cationic polymer like Q,-HEC inhibits the sulphur aggregation For Perfectamyl and carboxymethylcellulose no clear effect was measured Particularly for long-chain polymers, a distinct negative effect on the aggregation was found Steric hindrance, apparently, is an important factor in the aggregation process Upon increasing the sulphide loading rate, larger sulphur aggregates were formed while the opposite trend was observed for increasing salt concentrations In practice, therefore, a sulphide-oxidizing bioreactor should be operated at high loading rates to enhance the settleability of the sulphur sludge Keywords: Aggregation; Bacteria; Bioreactor; Natural colloids; Polymer adsorption; Sulphide; Waste water Introduction During the last decade, the application of the biological sulphur cycle in environmental technology has become increasingly popular [l-7] The processes involved concern the microbiological reduction of sulphate or sulphite, and the oxidation *Corresponding author Tel: +31(O) 8370 83339; Fax: +31(O) 8370 82108; e-mail: albert.jansen@algemeen.mt.wau.nl 0927-7765/96/$15.000 1996Elsevier Science B.V All rights reserved SSDI 0927-7765(95)01246-X of sulphide Under anaerobic conditions, sulphatereducing bacteria use sulphate as an electron acceptor, which results in the release of sulphide as one [S] In practice, this process of the end-products occurs for instance in high-rate UASB (Upflow Anaerobic Sludge Blanket) reactors when treating sulphate-containing waste waters from the paper or food industry [ 3,4] The sulphide formed has to be removed from the waste water because of its detrimental characteristics, e.g toxicity, corrosive 116 A Janssen et al./Colloids Surfaces B: Biointerfaces properties, oxygen demand and characteristic smell of rotten eggs [9] Under oxygen-limiting circumstances, bacteria of the genus thiobacillus oxidize sulphide to insoluble, elemental sulphur which can be separated from the liquid phase In this way, a reduction of the total amount of sulphur compounds in the discharged water can be achieved [ 2,101 The sulphur is a potentially valuable product which can be re-used, e.g in soil-bioleaching processes [ 61 Recently, a new biotechnological process based on the biological sulphur cycle was developed for removing SOZ from flue gases produced by coal combusting power plants [ 111 The process consists of three integrated reactors: the first reactor serves to scrub the SO2 present in the flue gas with an alkaline solution to form HSO;/SO:-, in the second reactor HSO;/SOiP is reduced to HS-, and in a third reactor sulphide is oxidized under oxygen-limiting circumstances to elemental sutphur These sulphur particles are separated in a tilted plate settler and the clean water can then be recirculated to the first reactor In case the sulphur particles are not completely removed, a part of the sulphur fraction will be recirculated and subsequently reduced in the anaerobic reactor An incomplete separation of sulphur particles therefore leads to (1) a double consumption of the required electron donor, e.g methanol, ethanol or hydrogen gas, and (2) increased sulphide levels in the anaerobic reactor which may cause inhibition of the metabolic processes taking place there [ 4,12,13] For these reasons, a highly effective sulphur removal step is essential for the successful application of the process In comparison with separation techniques such as flotation, filtration, extraction and membrane processes, plain sedimentation of sulphur particles undoubtedly represents the cheapest and technically most attractive method However, in order to be able to apply this method, the formation of easily settleable sulphur from the freshly formed but poor settling - sulphur particles is a prerequisite It is of importance therefore to improve our knowledge of the physical