Effect of floc surface morphology on membrane pollution prediction

doi:10.11949/0438-1157.20190828

Abstract

Abstract:

Existing prediction analysis of membrane fouling during coagulation-membrane filtration generally uses XDLVO theory to calculate the action energy of smooth interfaces, but the surface morphology of coagulated flocs will have a greater impact on the prediction results. In this study, a sinusoidal sphere model was used to simulate the surface of rough humic acid (HA) flocs. The interfacial interaction between coagulated flocs with different roughness and polyvinylidene fluoride (PVDF) membranes was quantitatively simulated by surface element integration (SEI) combined with XDLVO theory and compound Simpson rule. The interaction energies of smooth interfaces simulated by XDLVO theory are compared. The experimental results show that the model is suitable for the simulation of floc interfacial interaction energy in coagulation-membrane filtration system, and in the simulation process, the difference in interfacial interaction energy between 1—2 orders of magnitude due to the different roughness, and the fitting degree of rough flocs is better than that of completely smooth flocs and membrane fouling trend. High, that is, the introduction of floc surface morphology has a higher confidence in characterizing the trend of membrane fouling by the interaction between floc and membrane interface.

Key words: interface interaction, floc, XDLVO theory, membrane fouling, rough surface

摘要：

现有对混凝-膜过滤过程中膜污染的预测分析，一般采用XDLVO理论对光滑界面的作用能进行计算，但混凝絮体表面形态会对预测结果产生较大的影响。利用正弦波球体模型对粗糙腐殖酸(HA)絮体表面进行模拟，并通过表面元素积分法(SEI)结合XDLVO理论与复合辛普森规则，对不同粗糙程度的混凝絮体与聚偏氟乙烯(PVDF)膜的界面作用能进行量化模拟；并将结果与传统XDLVO理论模拟的光滑界面作用能进行了比较。实验结果表明，该模型适用于混凝-膜过滤体系中絮体界面作用能的模拟，同时在模拟过程中，由于粗糙度的不同会导致界面作用能在数值上存在1~2个数量级的差异；并且粗糙的絮体较完全光滑的絮体与膜污染趋势的拟合程度更高，即引入絮体表面形态对利用絮体与膜界面间相互作用能表征膜污染趋势的置信度更高。

关键词: 界面相互作用, 絮体, XDLVO理论, 膜污染, 粗糙表面

CLC Number:

TQ 028.8

Cheng FANG, Sheng YANG, Yun WU, Hongwei ZHANG, Jie WANG, Lutian WANG, Songze HAO. Effect of floc surface morphology on membrane pollution prediction[J]. CIESC Journal, 2020, 71(2): 715-723.

方乘, 杨盛, 吴云, 张宏伟, 王捷, 王鲁天, 郝松泽. 絮体表面形态对膜污染预测的影响[J]. 化工学报, 2020, 71(2): 715-723.

Figures/Tables 10

Fig.1 Ultrafiltration filtering device

Table 1 Surface energy parameters of three testing agents at 20℃

Liquid	γ^LW/(mJ/m²)	γ⁺/(mJ/m²)	γ^-/(mJ/m²)	γ^TOT/(mJ/m²)
pure water	21.8	25.5	25.5	72.8
glycerine	34.0	3.9	57.4	64.0
diiodomethane	50.8	0	0	50.8

Table 2 Particle size and fractal dimension of humic acid flocs at different pH

Floc morphology pH	Particle size/μm	Fractal dimension
5	114.1	2.1
6	112	2.14
7	118.8	2.13
8	112.5	2.05

Fig.2 3D model of rough flocs composed of different proportional parameters

Table 3 Contact angle and Zeta potential of PVDF film with humic acid at different pH

Sample	pH	Contact angle/(?)			Zeta potential, ξ/mV
Sample	pH	θ_w^①	θ_G^②	θ_D^③	Zeta potential, ξ/mV
PVDF	5	83.3±1.0	91.8±1.4	45.1±0.7	-18.0±1.8
	6	81.6±1.0	90.1±1.3	44.6±0.9	-20.4±0.9
	7	77.3±2.9	87.4±2.0	43.4±1.9	-22.0±1.4
	8	74.5±2.1	85.3±1.3	42.8±1.5	-27.2±0.5
HA	5	56.5±0.6	62.7±0.7	20.0±0.9	-20.9±0.8
	6	52.2±1.0	61.3±0.5	24.4±0.8	-33.3±0.5
	7	33.9±0.8	60.9±0.4	26.6±1.0	-37.4±0.3
	8	29.4±0.7	60.0±0.4	25.4±0.6	-40.9±0.9

Table 4 Surface tension parameters of PVDF film and humic acid at different pH

Sample	pH	γ⁺/(mJ/m²)	γ^-/ (mJ/m²)	γ^AB/ (mJ/m²)	γ^LW/ (mJ/m²)	γ^TOT/(mJ/m²)	?G_SWS/(mJ/m²)
PVDF	5	2.73	16.64	13.47	36.96	50.43	-17.17
	6	2.47	17.57	13.18	37.22	50.40	-16.04
	7	2.33	21.45	14.15	37.86	52.01	-10.30
	8	2.12	23.81	14.22	38.17	52.39	-7.00
HA	5	0.31	28.61	5.92	47.78	53.70	-4.69
	6	0.26	34.13	5.97	46.36	52.34	5.23
	7	0.97	62.98	15.65	45.57	61.21	38.26
	8	1.07	68.11	17.10	46.01	63.11	42.51

Table 5 Interaction energy of PVDF film-humic acid(PVDF-HA) in very close contact at different pH

pH	$U m l f L W$ /KT	$U m l f A B$ /KT	$U m l f E L$ /KT	$U m l f T O T$ /KT
5	-326.77	-1311.48	13.11	-1625.15
6	-340.79	-481.34	16.82	-805.33
7	-361.97	3767.87	49.35	3455.20
8	-354.68	4572.75	57.10	4275.17

Table 5 Interaction energy of PVDF film-humic acid(PVDF-HA) in very close contact at different pH

pH	$U m l f L W$ /KT	$U m l f A B$ /KT	$U m l f E L$ /KT	$U m l f T O T$ /KT
5	-326.77	-1311.48	13.11	-1625.15
6	-340.79	-481.34	16.82	-805.33
7	-361.97	3767.87	49.35	3455.20
8	-354.68	4572.75	57.10	4275.17

Table 6 Interaction energy of PVDF film-rough floc at very close contact at different pH

Sample	pH	$U m l f L W$ /KT	$U m l f A B$ /KT	$U m l f E L$ /KT	$U m l f T O T$ /KT
PVDF-rough flocs (λ=0.02，n=10)	5	-26.82	-102.15	12.43	-116.54
	6	-26.72	-35.39	21.79	-40.33
	7	-27.50	266.32	26.40	265.23
	8	-27.82	336.22	36.25	344.64
PVDF-rough flocs (λ=0.02，n=20)	5	-16.73	-40.94	11.09	-46.58
	6	-16.39	-12.64	19.87	-9.17
	7	-16.68	87.79	24.39	95.52
	8	-17.07	120.08	32.95	135.96
PVDF-rough flocs (λ=0.04，n=40)	5	-4.38	-24.69	3.31	-25.75
	6	-5.44	-6.55	5.59	-6.40
	7	-5.52	48.19	6.71	49.38
	8	-5.66	62.25	9.26	65.85

Table 6 Interaction energy of PVDF film-rough floc at very close contact at different pH

Sample	pH	$U m l f L W$ /KT	$U m l f A B$ /KT	$U m l f E L$ /KT	$U m l f T O T$ /KT
PVDF-rough flocs (λ=0.02，n=10)	5	-26.82	-102.15	12.43	-116.54
	6	-26.72	-35.39	21.79	-40.33
	7	-27.50	266.32	26.40	265.23
	8	-27.82	336.22	36.25	344.64
PVDF-rough flocs (λ=0.02，n=20)	5	-16.73	-40.94	11.09	-46.58
	6	-16.39	-12.64	19.87	-9.17
	7	-16.68	87.79	24.39	95.52
	8	-17.07	120.08	32.95	135.96
PVDF-rough flocs (λ=0.04，n=40)	5	-4.38	-24.69	3.31	-25.75
	6	-5.44	-6.55	5.59	-6.40
	7	-5.52	48.19	6.71	49.38
	8	-5.66	62.25	9.26	65.85

Fig.4 Fitting lines of interface interaction energy and membrane pollution trend under different roughness

Fig.3 Changes of relative flux with volume of leachate during humic acid microfiltration under different pH

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