The application of clays in various fields such as agriculture, mining, construction, petroleum derivatives recovery, waste water treatment and material reinforcements is closely related to their unique properties including small particle size with high surface negativity and hydrophilic nature. This work investigated the potential use of bentonite clay for treatment of meat rendering waste water by adsorption and utilizing the spent bentonite for bioplastic fillers. Models were applied to predict the organic adsorption and the sedimentation of clay particles in solution.
Various clays including bentonite are typically used in polymers to enhance composite properties at a very low level of loading. As bio-composite fillers, bentonite is modified with organics such as alkylammonium, amines and gelatin to improve compatibility with the matrix polymer. The use of these materials can be expensive and therefore, more attention has been directed to using waste sources such as meat processing waste. One type of waste, stickwater which contains approximately 4-5 % protein, 1-2 % fat and < 1 % minerals, could become a low-cost organic modifier for bentonite. Stickwater has a biological oxygen demand (BOD) of 150 000 mg/L, coloured, odorous and its treatment can be expensive.
Currently, stickwater is mostly dried and added to blood and bone meal in meat rendering plants. Where no further unit operations are carried out on stickwater, the stickwater must be treated to reduce the BOD. Depending on the scale of operation, a plant can produce up to 30 000 L of stickwater at 2-5 % solids per day. Waste water treatment costs NZ$ 0.90 per kg solids or approximately NZ$ 1350 per day. The main protein in stickwater is gelatin, a denatured form of collagen. Extracting protein from stickwater by adsorption can reduce the treatment charges by decreasing the BOD as well as producing organically-modified bentonite for use as a bio-composite filler.
Gelatin has a large numbers of side groups and can be easily crosslinked and adsorbed. Locally mined bentonite in the form of calcium and sodium bentonite, CaBt and NaBt respectively, costs NZ$ 20 per tonne and has a high gelatin adsorption capacity of up to 405 mg protein per gram bentonite (mg/g). For 30 000 kg stickwater, treatment using sodium bentonite costs around NZ$ 74 per day (this excludes operating and other material costs). Gelatin adsorption from stickwater gave a lower adsorption capacity of 246 mg/g due to competition between mineral and fat solutes with gelatin for binding sites on the adsorbent. Adsorption of solutes by bentonite occurred on the surface and within the interlayer spacings as revealed by X-ray diffraction (XRD) and thermogravimetric analysis (TGA).
Settling or sedimentation of particles from solution was studied to find an alternative to centrifugation for solid recovery after treatment. At 2 % protein concentration, settling was obtained for CaBt in gelatin at pH 3 whereas in stickwater, both CaBt and NaBt settled at pH 3. The highest clay recovery was obtained after overnight settling at 91 and 95 % for CaBt in gelatin and stickwater at pH 3. Conditions that favoured settling corresponded to low gelatin adsorption.
Bentonite consists of aluminosilicate layers and upon organic adsorption, intercalation and exfoliation can be achieved. As reinforcement materials, this leads to fillers with a very high aspect ratio in matrix polymers such as in bloodmeal-based plastic, also known as Novatein Thermoplastic Protein or NTP. In NTP, the highest improvement in tensile strength and Young’s modulus was at 23 % (11.45 MPa) and 17 % (727.45 MPa) at 0.5 parts stickwater-modified NaBt per hundred of bloodmeal (pphBM). For elongation at break, the highest increase of 23.5 % was achieved for unmodified NaBt at 0.5 pphBM. These improvements also correspond with enhancements in glass transition temperature (Tg) of composites and increased basal spacing (d-value) of bentonite. Transmission electron microscope (TEM) analysis confirmed the exfoliation of fillers for organically-modified bentonite. These observations, particularly at low filler dosage strongly suggest the formation of nanocomposites.
