Adsorption and kinetics of heavy metal (Zn) over biochars in solution

doi:10.16085/j.issn.1000-6613.2019-0302

Abstract

Abstract:

The biochars for adsorption of heavy metal (Zn) in solution were prepared by pyrolysis of water hyacinth (WH) and corn straw (CS) at 300—700℃. The effects of biomass species, pyrolysis temperature, initial pH and initial Zn(Ⅱ) concentration on Zn(Ⅱ) adsorption over biochars in solution were investigated, and the adsorption kinetics model was obtained based on the adsorption experiments. The results showed that the physicochemical characteristics were changed obviously with the increase of pyrolysis temperature, that is, the contents of volatile oxygen, hydrogen, O/C ratio and H/C ration of biochars decreased significantly; the contents of ash, fixed carbon and low heating value increased remarkably; the parameters of specific surface area, total pore volume, micro-pore volume, pH , and salt material of KCl increased significantly. The Zn(Ⅱ) adsorption capacity over biochar presented a trend of rapid increase followed by gradual stabilization or slight decline with increasing the initial pH, and the maximum equilibrium adsorption capacity of different biochars was obtained at pH=4—6. The Zn(Ⅱ) equilibrium adsorption capacity over biochar presented rapid linear increase with increasing initial Zn(Ⅱ) concentration while the initial Zn(Ⅱ) concentration was less than 30mg/L, however, the increase trend of Zn(Ⅱ) equilibrium adsorption capacity slowed down while the initial Zn(Ⅱ) concentration was larger than 30mg/L. The Zn(Ⅱ) equilibrium adsorption capacity increased gradually with increasing pyrolysis temperature at the same initial Zn(Ⅱ) concentration, and the Zn(Ⅱ) equilibrium adsorption capacity over WH biochar was significantly larger than that over CS biochar prepared at the same pyrolysis temperature. The adsorption kinetics of Zn(Ⅱ) on two biochars in solution was in accordance with Lagergren pseudo-second-order model, which was mainly controlled by chemical adsorption. The adsorption mechanism of Zn(Ⅱ) on WHC and CSC mainly involved the complexation of oxygen-containing functional groups and the precipitation of inorganic salt ions.

Key words: biochar, heavy metal, zinc, adsorption, kinetics

摘要：

在300～700℃下制备了水葫芦炭和玉米秸秆炭，研究了生物质种类、热解温度、溶液初始pH和Zn(Ⅱ)初始浓度对两种生物炭吸附溶液中Zn(Ⅱ)的影响，并结合吸附过程曲线拟合获得了吸附动力学模型。结果表明：随着热解温度的升高，生物炭理化特性发生显著变化，生物炭的挥发分、氧含量、氢含量以及O/C和H/C显著降低，而固定碳、灰分和热值显著升高，生物炭的比表面积、总孔容、微孔容、pH以及KCl等盐类物质均得到了显著增加。随着溶液初始pH增加，生物炭对Zn(Ⅱ)的吸附能力呈现先快速增加然后逐步趋于稳定或稍有下降的趋势，不同生物炭的最大平衡吸附量出现在pH=4~6之间。Zn(Ⅱ)初始浓度<30mg/L时，生物炭对Zn(Ⅱ)平衡吸附量随溶液Zn(Ⅱ)初始浓度的增加呈线性快速增长，而当Zn(Ⅱ)初始浓度>30mg/L，其平衡吸附量增长趋势变缓。在相同Zn(Ⅱ)初始浓度下，随着热解温度的提高，生物炭对溶液中Zn(Ⅱ)平衡吸附量逐渐提高，且在同一热解温度下制备的水葫芦炭对Zn(Ⅱ)的平衡吸附量显著高于玉米秸秆炭。两种生物炭对溶液Zn(Ⅱ)的吸附符合Lagergren准二级动力学模型，其吸附过程均受化学吸附控制，水葫芦炭和玉米秸秆炭对Zn(Ⅱ)吸附机制主要包括含氧官能团的络合作用和无机盐离子的沉淀作用。

关键词: 生物炭, 重金属, 锌, 吸附, 动力学

CLC Number:

Yuxuan WANG,Hong WANG,Ping LU. Adsorption and kinetics of heavy metal (Zn) over biochars in solution[J]. Chemical Industry and Engineering Progress, 2019, 38(11): 5142-5150.

王昱璇,王红,卢平. 生物炭对重金属（Zn）的吸附特性及动力学[J]. 化工进展, 2019, 38(11): 5142-5150.

Figures/Tables 8

References 33

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样品	工业分析/%				元素分析/%					Q _net,ad	H/C	O/C	(O+N)/C
样品	M_ad	V_ad	FC_ad	A_ad	C_ad	H_ad	O_ad ^①	N_ad	S_ad	/MJ·kg^-1	H/C	O/C	(O+N)/C
CS	13.14	64.68	15.82	6.36	40.71	5.64	33.59	0.50	0.06	14.11	0.14	0.83	0.84
CSC300	—	42.67	43.57	13.76	58.24	4.70	22.75	0.49	0.06	22.76	0.08	0.39	0.40
CSC400	—	23.08	59.78	17.13	64.89	4.01	13.48	0.48	0.01	20.58	0.06	0.21	0.22
CSC500	—	13.09	66.23	20.68	67.77	3.06	8.01	0.47	0.01	22.84	0.05	0.12	0.13
CSC600	—	8.41	70.36	21.23	68.96	2.56	6.78	0.46	0.01	25.37	0.04	0.10	0.11
CSC700	—	6.99	70.34	22.68	69.20	1.84	5.79	0.43	0.06	24.73	0.03	0.08	0.09
WH	13.93	60.79	12.84	12.45	36.94	5.63	29.56	1.33	0.16	10.52	0.15	0.80	0.84
WHC300	—	45.55	33.12	21.32	50.15	4.34	21.86	2.22	0.11	19.20	0.09	0.44	0.48
WHC400	—	29.67	42.59	27.75	52.31	3.72	13.96	2.19	0.07	19.71	0.07	0.27	0.31
WHC500	—	18.93	48.50	32.57	51.89	2.79	10.59	2.05	0.11	18.95	0.05	0.20	0.24
WHC600	—	13.66	51.13	35.20	53.24	1.98	7.65	1.92	0.01	19.21	0.04	0.14	0.18
WHC700	—	9.22	54.82	35.96	55.57	1.66	5.20	1.51	0.10	19.35	0.03	0.09	0.12

样品	工业分析/%				元素分析/%					Q _net,ad	H/C	O/C	(O+N)/C
样品	M_ad	V_ad	FC_ad	A_ad	C_ad	H_ad	O_ad ^①	N_ad	S_ad	/MJ·kg^-1	H/C	O/C	(O+N)/C
CS	13.14	64.68	15.82	6.36	40.71	5.64	33.59	0.50	0.06	14.11	0.14	0.83	0.84
CSC300	—	42.67	43.57	13.76	58.24	4.70	22.75	0.49	0.06	22.76	0.08	0.39	0.40
CSC400	—	23.08	59.78	17.13	64.89	4.01	13.48	0.48	0.01	20.58	0.06	0.21	0.22
CSC500	—	13.09	66.23	20.68	67.77	3.06	8.01	0.47	0.01	22.84	0.05	0.12	0.13
CSC600	—	8.41	70.36	21.23	68.96	2.56	6.78	0.46	0.01	25.37	0.04	0.10	0.11
CSC700	—	6.99	70.34	22.68	69.20	1.84	5.79	0.43	0.06	24.73	0.03	0.08	0.09
WH	13.93	60.79	12.84	12.45	36.94	5.63	29.56	1.33	0.16	10.52	0.15	0.80	0.84
WHC300	—	45.55	33.12	21.32	50.15	4.34	21.86	2.22	0.11	19.20	0.09	0.44	0.48
WHC400	—	29.67	42.59	27.75	52.31	3.72	13.96	2.19	0.07	19.71	0.07	0.27	0.31
WHC500	—	18.93	48.50	32.57	51.89	2.79	10.59	2.05	0.11	18.95	0.05	0.20	0.24
WHC600	—	13.66	51.13	35.20	53.24	1.98	7.65	1.92	0.01	19.21	0.04	0.14	0.18
WHC700	—	9.22	54.82	35.96	55.57	1.66	5.20	1.51	0.10	19.35	0.03	0.09	0.12

样品	S _BET /m²·g^-1	S _D-R /m²·g^-1	V _T /mm³·g^-1	V _M /mm³·g^-1	X /%	D _a /nm
CS	1.54	2.39	2.9	0.9	31.30	7.53
CS300	6.80	11.34	9.4	4.3	45.99	5.53
CS400	10.51	16.57	11.7	5.9	50.43	4.45
CS500	13.88	20.82	13.1	7.4	56.49	3.78
CS600	60.50	81.25	61.2	25.1	41.01	4.05
CS700	20.92	30.35	21.1	10.8	51.18	4.03
WH	2.26	3.36	5.8	1.2	20.83	10.27
WH300	2.79	5.33	8.2	1.9	23.17	11.76
WH400	3.80	5.69	8.3	2.0	24.10	8.74
WH500	8.19	12.13	16.9	4.2	24.85	8.24
WH600	37.46	52.85	39.2	18.8	47.96	4.19
WH700	53.74	78.14	43.6	27.8	63.76	3.25

样品	S _BET /m²·g^-1	S _D-R /m²·g^-1	V _T /mm³·g^-1	V _M /mm³·g^-1	X /%	D _a /nm
CS	1.54	2.39	2.9	0.9	31.30	7.53
CS300	6.80	11.34	9.4	4.3	45.99	5.53
CS400	10.51	16.57	11.7	5.9	50.43	4.45
CS500	13.88	20.82	13.1	7.4	56.49	3.78
CS600	60.50	81.25	61.2	25.1	41.01	4.05
CS700	20.92	30.35	21.1	10.8	51.18	4.03
WH	2.26	3.36	5.8	1.2	20.83	10.27
WH300	2.79	5.33	8.2	1.9	23.17	11.76
WH400	3.80	5.69	8.3	2.0	24.10	8.74
WH500	8.19	12.13	16.9	4.2	24.85	8.24
WH600	37.46	52.85	39.2	18.8	47.96	4.19
WH700	53.74	78.14	43.6	27.8	63.76	3.25

生物炭	C ₀ /mg·g^-1	实验平衡吸附量Q _e1/mg·g^-1	一级动力学吸附模型			二级动力学吸附模型
生物炭	C ₀ /mg·g^-1	实验平衡吸附量Q _e1/mg·g^-1	Q _e2 /mg·g^-1	K ₁ /h^-1	R ²	Q _e2 /mg·g^-1	K ₂ /g·mg^-1·h^-1	R ²
CS300	30	21.82	19.44	1.458	0.9026	22.12	0.064	0.9983
CS400	30	20.35	17.99	1.877	0.8773	20.57	0.077	0.9978
CS500	30	24.12	22.34	2.104	0.9060	23.69	0.073	0.9979
CS600	30	21.41	20.47	2.979	0.9748	21.47	0.214	0.9998
CS700	30	22.06	20.88	3.639	0.9738	21.97	0.215	0.9994
WH300	30	24.66	23.19	2.018	0.9588	24.88	0.107	0.9997
WH400	30	28.05	25.77	1.924	0.8934	28.48	0.075	0.9996
WH500	30	30.80	28.58	2.124	0.9488	31.05	0.080	0.9996
WH600	30	35.55	32.42	1.764	0.8782	36.03	0.053	0.9995
WH700	30	44.95	41.35	1.409	0.9220	45.84	0.036	0.9997